build: Use portable sed syntax in src/Makefile.am

Replace non-portable \+ with \{, \} to avoid build failure on systems
not using GNU sed.

https://bugzilla.gnome.org/show_bug.cgi?id=792280

src/Makefile.am

@@ -750,7 +750,7 @@ $(if $(findstring unstable,$1),unstable,stable)
 endef
 
 define protoname
-$(shell echo $1 | sed 's/\([a-z\-]\+\)-[a-z]\+-v[0-9]\+/\1/')
+$(shell echo $1 | sed 's/\([a-z\-]\{1,\}\)-[a-z]\{1,\}-v[0-9]\{1,\}/\1/')
 endef
 
 %-protocol.c : $(WAYLAND_PROTOCOLS_DATADIR)/$$(call protostability,$$*)/$$(call protoname,$$*)/$$*.xml