checksrc: fix uninitialized variable warning

The loop doesn't need to be executed without a file argument.

Closes https://github.com/curl/curl/pull/4444
diff --git a/lib/checksrc.pl b/lib/checksrc.pl
index 965f0ba..b2cfa83 100755
--- a/lib/checksrc.pl
+++ b/lib/checksrc.pl
@@ -176,7 +176,7 @@
 
 $file = shift @ARGV;
 
-while(1) {
+while(defined $file) {
 
     if($file =~ /-D(.*)/) {
         $dir = $1;