[libmariadb] Fix the issue of iconv search failure under Linux (#39136)

Fix one of https://github.com/microsoft/vcpkg/issues/32398 issue.
```
CMake Error at mariadb_config/CMakeLists.txt:51 (GET_LIB_NAME):
  GET_LIB_NAME Function invoked with incorrect arguments for function named:
  GET_LIB_NAME
```
Fixed the issue that the search for iconv in Linux did not obtain the
value of the macro variable Iconv_LIBRARIES, causing the function input
parameter check to fail.

- [X] Changes comply with the [maintainer
guide](https://github.com/microsoft/vcpkg-docs/blob/main/vcpkg/contributing/maintainer-guide.md).
- [ ] ~~SHA512s are updated for each updated download.~~
- [ ] ~~The "supports" clause reflects platforms that may be fixed by
this new version.~~
- [ ] ~~Any fixed [CI
baseline](https://github.com/microsoft/vcpkg/blob/master/scripts/ci.baseline.txt)
entries are removed from that file.~~
- [ ] ~~Any patches that are no longer applied are deleted from the
port's directory.~~
- [X] The version database is fixed by rerunning `./vcpkg x-add-version
--all` and committing the result.
- [X] Only one version is added to each modified port's versions file.

Usage test pass with following triplets:

```
x64-linux
```

ports/libmariadb/fix-iconv.patch

@@ -33,8 +33,8 @@ index 5bbf36b..70e619b 100644
  IF(UNIX AND NOT APPLE)
 -  IF(ICONV_EXTERNAL)
 -    GET_LIB_NAME(${ICONV_LIBRARIES} LIB_OUT)
-+  IF(WITH_ICONV)
-+    GET_LIB_NAME(${Iconv_LIBRARIES} LIB_OUT)
++  IF(WITH_ICONV AND NOT Iconv_IS_BUILT_IN)
++    GET_LIB_NAME("${Iconv_LIBRARIES}" LIB_OUT)
      SET(extra_dynamic_LDFLAGS "${extra_dynamic_LDFLAGS} ${LIB_OUT}")
    ENDIF()
  ENDIF()

ports/libmariadb/vcpkg.json

@@ -1,7 +1,7 @@
 {
   "name": "libmariadb",
   "version-semver": "3.3.1",
-  "port-version": 3,
+  "port-version": 4,
   "description": "MariaDB Connector/C is used to connect C/C++ applications to MariaDB and MySQL databases",
   "homepage": "https://github.com/MariaDB/mariadb-connector-c",
   "license": "LGPL-2.1-or-later",

versions/baseline.json

@@ -4638,7 +4638,7 @@
     },
     "libmariadb": {
       "baseline": "3.3.1",
-      "port-version": 3
+      "port-version": 4
     },
     "libmaxminddb": {
       "baseline": "1.9.1",

versions/l-/libmariadb.json

@@ -1,5 +1,10 @@
 {
   "versions": [
+    {
+      "git-tree": "e0c869b45c183f7486a6da4785259a949bd18b25",
+      "version-semver": "3.3.1",
+      "port-version": 4
+    },
     {
       "git-tree": "c3d524a86a6280626f4955396e2dc21ad5f8be35",
       "version-semver": "3.3.1",