MacOS: Add NFD_ClearError() definition (#88)

This function was overlooked in the original implementation, and trying to call it would lead to a linker error.

src/nfd_cocoa.m

@@ -180,6 +180,10 @@ static nfdresult_t CopyUtf8String(const char* utf8Str, nfdnchar_t** out) {
     return g_errorstr;
 }
 
+void NFD_ClearError(void) {
+    NFDi_SetError(NULL);
+}
+
 void NFD_FreePathN(nfdnchar_t* filePath) {
     NFDi_Free((void*)filePath);
 }