kbuild: add a missing line for help message

The help message line for building the source RPM package was missing.
Added it.

Signed-off-by: Jun ASAKA <JunASAKA@zzy040330.moe>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
diff --git a/scripts/Makefile.package b/scripts/Makefile.package
index 539e9f7..525a282 100644
--- a/scripts/Makefile.package
+++ b/scripts/Makefile.package
@@ -158,6 +158,7 @@
 PHONY += help
 help:
 	@echo '  rpm-pkg             - Build both source and binary RPM kernel packages'
+	@echo '  srcrpm-pkg          - Build only the source kernel RPM package'
 	@echo '  binrpm-pkg          - Build only the binary kernel RPM package'
 	@echo '  deb-pkg             - Build both source and binary deb kernel packages'
 	@echo '  bindeb-pkg          - Build only the binary kernel deb package'