Add Arch support for conf-gmp

Author: jmid

packages/conf-gmp/conf-gmp.5/opam

@@ -31,6 +31,7 @@ depexts: [
   ["gmp" "gmp-devel"] {os-distribution = "ol"}
   ["gmp"] {os = "openbsd"}
   ["gmp"] {os = "freebsd"}
+  ["gmp"] {os-distribution = "arch"}
   ["gmp-dev"] {os-distribution = "alpine"}
   ["gmp-devel"] {os-family = "suse" | os-family = "opensuse"}
   ["gmp"] {os = "win32" & os-distribution = "cygwinports"}