vpx_frame_buffer.h: add doxygen \file block

the remainder of the documentation will not be included in the output
unless the file itself is documented

Change-Id: I5a83a6c41cdfbf2976da288e4b70bd04002725f2

vpx/vpx_frame_buffer.h

@@ -11,6 +11,10 @@
 #ifndef VPX_VPX_FRAME_BUFFER_H_
 #define VPX_VPX_FRAME_BUFFER_H_
 
+/*!\file
+ * \brief Describes the decoder external frame buffer interface.
+ */
+
 #ifdef __cplusplus
 extern "C" {
 #endif