CMake: Add Libs.private to Libs if installing static only

pkg-config --libs --static raylib and pkg-config --libs raylib
should give the same result if there is no shared raylib installed.

raylib.pc.in

@@ -7,7 +7,7 @@ Name: raylib
 Description: Simple and easy-to-use library to learn videogames programming
 URL: http://github.com/raysan5/raylib
 Version: @PROJECT_VERSION@
-Libs: -L${libdir} -lraylib
+Libs: -L${libdir} -lraylib @PKG_CONFIG_LIBS_EXTRA@
 Libs.private: @PKG_CONFIG_LIBS_PRIVATE@
 Requires.private: @GLFW_PKG_DEPS@
 Cflags: -I${includedir}