Makefile: break headers and source into 2 vars

Split the headers and source into 2 variables. Headers are
marked noinst so they don't get automatically installed on
"make install". Instead they will be installed by a custom
Makefile target, "make install-headers".

Makefile.am

@@ -59,3 +59,6 @@ install-library:
 	cd src && $(MAKE) $@
 	cd rust && $(MAKE) $@
 	$(INSTALL) libsuricata-config "$(DESTDIR)$(bindir)/libsuricata-config"
+
+install-headers:
+	cd src && $(MAKE) $@