external/gpgmepp: Avoid overloaded utf8_to_wchar in C code

> w32-util.c(176,1): error: conflicting types for 'utf8_to_wchar'
> utf8_to_wchar (const char *string)
> ^
> workdir/UnpackedTarball/libgpg-error/src\gpg-error.h(1109,10): note: previous declaration is here
> wchar_t *utf8_to_wchar (const char *string, size_t length, size_t *retlen);
>          ^

with clang-cl on Windows, while in a case like this where there is only one
definition, the mismatching declaration merely gets warned about by MSVC with
"warning C4029: declared formal parameter list different from definition".  (And
on non-Windows that w32-util.c apparently doesn't get compiled at all.)

Change-Id: I76cfc3ec086325c527c04dbe0e8341cb9b775c50
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/100091
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

external/gpgmepp/UnpackedTarball_gpgmepp.mk

@@ -29,5 +29,6 @@ $(eval $(call gb_UnpackedTarball_add_patches,gpgmepp, \
     external/gpgmepp/gcc9.patch \
     external/gpgmepp/ubsan.patch \
     external/gpgmepp/c++20.patch \
+    external/gpgmepp/clang-cl.patch \
 ))
 # vim: set noet sw=4 ts=4:

external/gpgmepp/clang-cl.patch

@@ -0,0 +1,20 @@
+--- src/w32-util.c
++++ src/w32-util.c
+@@ -173,7 +173,7 @@
+    NULL; caller may use GetLastError to get the actual error number.
+    Calling this function with STRING set to NULL is not defined. */
+ static wchar_t *
+-utf8_to_wchar (const char *string)
++utf8_to_wchar_ (const char *string)
+ {
+   int n;
+   wchar_t *result;
+@@ -206,7 +206,7 @@
+   if (!string)
+     return NULL;
+ 
+-  return utf8_to_wchar (string);
++  return utf8_to_wchar_ (string);
+ }
+ 
+