Include config.h in mini-gmp.c, if present

* libguile/mini-gmp.c: Add config.h include.  Handles the case in which
gnulib provides some part of the standard library of mini-gmp, as is
apparently the case on FreeBSD for example.  Thanks to RhodiumToad for
the report and fix.

libguile/mini-gmp.c

@@ -41,6 +41,10 @@ see https://www.gnu.org/licenses/.  */
    mpn/generic/sbpi1_div_qr.c, mpn/generic/sub_n.c,
    mpn/generic/submul_1.c. */
 
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
 #include <assert.h>
 #include <ctype.h>
 #include <limits.h>