C bindings: Include missing wchar header in tag_c.h (#1273)

In C++, wchar_t is a built-in fundamental type, but in C, it is not.
Compilation with MinGW will fail without a header included which
defines wchar_t (wchar.h or stddef.h). The other compilers used in our
CI (gcc on Ubuntu, clang on macOS and MSVC on Windows) seem to
know wchar_t without an include.

Author: ufleisch

bindings/c/tag_c.h

@@ -43,6 +43,7 @@ extern "C" {
 #define TAGLIB_C_EXPORT
 #endif
 
+#include <wchar.h>
 #ifdef _MSC_VER
 /* minwindef.h contains typedef int BOOL */
 #include <windows.h>