mesa: update comment for UniformBufferSize to indicate size is in bytes

Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/mesa/main/mtypes.h

@@ -2560,7 +2560,7 @@ struct gl_uniform_block
    GLuint Binding;
 
    /**
-    * Minimum size of a buffer object to back this uniform buffer
+    * Minimum size (in bytes) of a buffer object to back this uniform buffer
     * (GL_UNIFORM_BLOCK_DATA_SIZE).
     */
    GLuint UniformBufferSize;