gl_buffer_cache: Make constructor explicit

Implicit conversions during construction isn't desirable here.

src/video_core/renderer_opengl/gl_buffer_cache.h

@@ -31,7 +31,7 @@ struct CachedBufferEntry final {
 
 class OGLBufferCache final : public RasterizerCache<std::shared_ptr<CachedBufferEntry>> {
 public:
-    OGLBufferCache(size_t size);
+    explicit OGLBufferCache(size_t size);
 
     GLintptr UploadMemory(Tegra::GPUVAddr gpu_addr, size_t size, size_t alignment = 4,
                           bool cache = true);