regex: avoid compiler warning Although glibc leaks strcasecmp through <string.h>, this behavior is not required by POSIX, and HP NonStop needs <strings.h> * lib/regex.c (includes): Include <strings.h>, for use of strcasecmp in regcomp.c. Reported by Jachim Schmitz. Signed-off-by: Eric Blake <eblake@redhat.com>