Include <strings.h> for the strcasecmp function

Otherwise, an implicit function declaration is the result, and the
code may fail to compile with future compilers.

Submitted upstream:

  <https://lore.kernel.org/linux-efi/87pmcllll9.fsf@oldenburg.str.redhat.com/>

diff --git a/efi-updatevar.c b/efi-updatevar.c
index 4247105..033d938 100644
--- a/efi-updatevar.c
+++ b/efi-updatevar.c
@@ -11,6 +11,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <strings.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <unistd.h>