ctags: Generate tags for headers, i.e. function prototypes

From time to time it's handy to jump directly to a function prototype in
a header. However, generating tags for headers is disabled by default in
ctags, let's enable it by using --c-kinds=+p.
Signed-off-by: NErik Skultety <eskultet@redhat.com>

.ctags

@@ -3,3 +3,4 @@
 --exclude=*.html
 --exclude=*.html.in
 --langmap=c:+.h.in
+--c-kinds=+p