update gperf call to work with gperf-3.1

https://bugs.gentoo.org/604802

--- a/libcap/Makefile
+++ b/libcap/Makefile
@@ -44,11 +44,12 @@
 	./_makenames > cap_names.h
 
 $(GPERF_OUTPUT): cap_names.list.h
-	(printf "%b" "struct __cap_token_s { const char *name; int index; };\n%{\nconst struct __cap_token_s *__cap_lookup_name(const char *, unsigned int);\n%}\n%%\n"; \
+	(printf "%b" "struct __cap_token_s { const char *name; int index; };\n%%\n"; \
 	$(SED) -e 's:["{}]::g' -e 's:,$$::' $<) | \
 	gperf \
 		--ignore-case \
 		--language=ANSI-C \
+		--includes \
 		--readonly \
 		--null-strings \
 		--global-table \