pkg-config: Empty Requires.private on shared-only build

If user doesn't build the static library,
`pkg-config --static --libs raylib` should be equivalent to
`pkg-config --libs raylib`.

raylib.pc.in

@@ -8,6 +8,6 @@ Description: Simple and easy-to-use library to learn videogames programming
 URL: http://github.com/raysan5/raylib
 Version: @PROJECT_VERSION@
 Libs: -L${libdir} -lraylib
-Libs.private:@PKG_CONFIG_LIBS_PRIVATE@
+Libs.private: @PKG_CONFIG_LIBS_PRIVATE@
 Requires.private:
 Cflags: -I${includedir}