Change libgmp url to my mirror (#384)

Author: ryanking13

packages/libgmp/meta.yaml

@@ -5,7 +5,7 @@ package:
     - library
     - static_library
 source:
-  url: https://ftp.gnu.org/gnu/gmp/gmp-6.3.0.tar.xz
+  url: https://github.com/ryanking13/lib-mirror/releases/download/1.0.0/gmp-6.3.0.tar.xz
   sha256: a3c2b80201b89e68616f4ad30bc66aee4927c3ce50e33929ca819d5c43538898
 
 build: