stdlib: Add SDL_utf8strnlen

src/dynapi/SDL_dynapi_overrides.h

@@ -874,3 +874,4 @@
 #define SDL_HasLSX SDL_HasLSX_REAL
 #define SDL_HasLASX SDL_HasLASX_REAL
 #define SDL_RenderGetD3D12Device SDL_RenderGetD3D12Device_REAL
+#define SDL_utf8strnlen SDL_utf8strnlen_REAL

src/dynapi/SDL_dynapi_procs.h

@@ -951,3 +951,4 @@ SDL_DYNAPI_PROC(SDL_bool,SDL_HasLASX,(void),(),return)
 #ifdef __WIN32__
 SDL_DYNAPI_PROC(ID3D12Device*,SDL_RenderGetD3D12Device,(SDL_Renderer *a),(a),return)
 #endif
+SDL_DYNAPI_PROC(size_t,SDL_utf8strnlen,(const char *a, size_t b),(a,b),return)