syscall: prevent redundant newline from empty $extraimports

Currently mksyscall script produces one redundant newline
when $extraimports is empty, leading to a gofmt warning
reported by "git codereview change".

Change-Id: I57b0fa040347375698973e777285deded16ab62e
Reviewed-on: https://go-review.googlesource.com/c/go/+/587877
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: panjf2000

src/syscall/mksyscall.pl

@@ -33,6 +33,7 @@
 my $libc = 0;
 my $tags = "";  # build tags
 my $newtags = ""; # new style build tags
+my $stdimports = 'import "unsafe"';
 my $extraimports = "";
 
 if($ARGV[0] eq "-b32") {
@@ -390,6 +391,10 @@ ($)
 	exit 1;
 }
 
+if($extraimports ne "") {
+    $stdimports .= "\n$extraimports";
+}
+
 # TODO: this assumes tags are just simply comma separated. For now this is all the uses.
 $newtags = $tags =~ s/,/ && /r;
 
@@ -401,8 +406,7 @@ ($)
 
 package syscall
 
-import "unsafe"
-$extraimports
+$stdimports
 
 $text
 EOF