glapi: Use GetProcAddress instead of dlsym on Windows.

This patch fixes this MinGW build error.

glapi_gentable.c: In function '_glapi_create_table_from_handle':
glapi_gentable.c:123:9: error: implicit declaration of function 'dlsym' [-Werror=implicit-function-declaration]
         *procp = dlsym(handle, symboln);
         ^

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Acked-by: Brian Paul <brianp@vmware.com>

src/mapi/glapi/gen/gl_gentable.py

@@ -134,7 +134,11 @@ body_template = """
     if(!disp->%(name)s) {
         void ** procp = (void **) &disp->%(name)s;
         snprintf(symboln, sizeof(symboln), "%%s%(entry_point)s", symbol_prefix);
+#ifdef _WIN32
+        *procp = GetProcAddress(handle, symboln);
+#else
         *procp = dlsym(handle, symboln);
+#endif
     }
 """