gl_shader_disk_cache: Remove #pragma once from cpp file

This is only necessary in headers. Silences a warning with clang.

src/video_core/renderer_opengl/gl_shader_disk_cache.cpp

@@ -2,8 +2,6 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
-#pragma once
-
 #include <cstring>
 #include <fmt/format.h>
 #include <lz4.h>