gl_buffer_cache: Make GetHandle() a const member function

GetHandle() internally calls GetHandle() on the stream_buffer instance,
which is a const member function, so this can be made const as well.

src/video_core/renderer_opengl/gl_buffer_cache.cpp

@@ -77,7 +77,7 @@ void OGLBufferCache::Unmap() {
     stream_buffer.Unmap(buffer_offset - buffer_offset_base);
 }
 
-GLuint OGLBufferCache::GetHandle() {
+GLuint OGLBufferCache::GetHandle() const {
     return stream_buffer.GetHandle();
 }
 

src/video_core/renderer_opengl/gl_buffer_cache.h

@@ -41,7 +41,7 @@ public:
     void Map(size_t max_size);
     void Unmap();
 
-    GLuint GetHandle();
+    GLuint GetHandle() const;
 
 protected:
     void AlignBuffer(size_t alignment);