[Ada] Update the inactive GMP variant of Big_Integers

[Pierre-Marie de Rodat via Gcc-patches]

The GMP variant of Ada.Numerics.Big_Numbers.Big_Integers is currently
not used, but since we keep it, it seems worth to keep it up-to-date
with respect to the corresponding spec.

Part of providing a gdb pretty-printer for Big_Integer objects.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * libgnat/a-nbnbin__gmp.adb (From_String): Fix predicate
        mismatch between subprogram declaration and body.
diff --git a/gcc/ada/libgnat/a-nbnbin__gmp.adb b/gcc/ada/libgnat/a-nbnbin__gmp.adb
--- a/gcc/ada/libgnat/a-nbnbin__gmp.adb
+++ b/gcc/ada/libgnat/a-nbnbin__gmp.adb
@@ -327,7 +327,7 @@ package body Ada.Numerics.Big_Numbers.Big_Integers is
    -- From_String --
    -----------------
 
-   function From_String (Arg : String) return Big_Integer is
+   function From_String (Arg : String) return Valid_Big_Integer is
       function mpz_set_str
         (this : access mpz_t;
          str  : System.Address;