tools: bootconfig: Regex enclosed with quotes to make syntax highlight proper

As suggested, changed the square brackets escaping to quote the whole Regex
class.

Link: https://lore.kernel.org/all/20250709030141.27483-1-unixbhaskar@gmail.com/

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Author: unixbhaskar

tools/bootconfig/test-bootconfig.sh

@@ -167,8 +167,8 @@ echo > $INITRD
 
 xpass $BOOTCONF -a $TEMPCONF $INITRD
 $BOOTCONF $INITRD > $OUTFILE
-xfail grep -q val[[:space:]] $OUTFILE
-xpass grep -q val2[[:space:]] $OUTFILE
+xfail grep -q 'val[[:space:]]' $OUTFILE
+xpass grep -q 'val2[[:space:]]' $OUTFILE
 
 echo "=== expected failure cases ==="
 for i in samples/bad-* ; do