include/linux/string_helpers.h: add linux/string.h for strlen()

linux/string_helpers.h uses strlen(), so include the correponding header.
Otherwise we get a compilation error if it's not also included by whoever
included the helper.

Link: https://lkml.kernel.org/r/20211005212634.3223113-1-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/string_helpers.h

@@ -4,6 +4,7 @@
 
 #include <linux/bits.h>
 #include <linux/ctype.h>
+#include <linux/string.h>
 #include <linux/types.h>
 
 struct file;