Doxyfile.in: enable MACRO_EXPANSION

this avoids littering function definitions in the documentation with
"PROTOBUF_C_API".

Doxyfile.in

@@ -1902,7 +1902,7 @@ ENABLE_PREPROCESSING   = YES
 # The default value is: NO.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-MACRO_EXPANSION        = NO
+MACRO_EXPANSION        = YES
 
 # If the EXPAND_ONLY_PREDEF and MACRO_EXPANSION tags are both set to YES then
 # the macro expansion is limited to the macros specified with the PREDEFINED and