Improve ctags invocation in Makefile

Adding .function was necessary, as otherwise ctags would have no idea what to
do with those files.

Adding .h may not be necessary, as by default ctags considers them C++ which
is probably good enough, but since we're tuning the mapping anyway...

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

Makefile

@@ -125,7 +125,7 @@ endif
 ## Editor navigation files
 C_SOURCE_FILES = $(wildcard include/*/*.h library/*.[hc] programs/*/*.[hc] tests/suites/*.function)
 tags: $(C_SOURCE_FILES)
-	ctags -o $@ $(C_SOURCE_FILES)
+	ctags -o $@ --langmap=c:+.h.function $(C_SOURCE_FILES)
 TAGS: $(C_SOURCE_FILES)
 	etags -o $@ $(C_SOURCE_FILES)
 GPATH GRTAGS GSYMS GTAGS: $(C_SOURCE_FILES)