demos: Use WinMain instead of wmain on Win32

We're going to want to build as a GUI app, so we need
to use real WinMain on Windows for this.

Signed-off-by: Joshua Ashton <joshua@froggi.es>

demos/gears.c

@@ -878,7 +878,11 @@ static int cxg_main(void)
 }
 
 #ifdef _WIN32
-int wmain(void)
+int WINAPI WinMain(
+    HINSTANCE hInstance,
+    HINSTANCE hPrevInstance,
+    LPSTR lpCmdLine,
+    int nCmdShow)
 #else
 int main(void)
 #endif

demos/triangle.c

@@ -393,7 +393,11 @@ static int cxt_main(void)
 }
 
 #ifdef _WIN32
-int wmain(void)
+int WINAPI WinMain(
+    HINSTANCE hInstance,
+    HINSTANCE hPrevInstance,
+    LPSTR lpCmdLine,
+    int nCmdShow)
 #else
 int main(void)
 #endif