Fix undefined variable in scripts/config.pl

The variable $config_file was being referenced without being defined in the
script config.pl.
diff --git a/scripts/config.pl b/scripts/config.pl
index 5e1865d..d8d6a20 100755
--- a/scripts/config.pl
+++ b/scripts/config.pl
@@ -5,6 +5,7 @@
 use warnings;
 use strict;
 
+my $config_file = "include/mbedtls/config.h";
 my $usage = <<EOU;
 $0 [-f <file>] [set <symbol> <value> | unset <symbol> | full | realfull]
 
@@ -58,8 +59,6 @@
 PLATFORM_[A-Z0-9]+_ALT
 );
 
-my $config_file = "include/mbedtls/config.h";
-
 # get -f option
 if (@ARGV >= 2 && $ARGV[0] eq "-f") {
     shift; # -f