Include <strings.h> directly where needed [minor]

Other source files that use strcase*() functions #include <strings.h>
themselves, so do so for this source file too. (<strings.h> is often
a byproduct of <string.h> but POSIX doesn't require that.)

Author: jmarshall

csq.c

@@ -149,6 +149,7 @@
 #include <errno.h>
 #include <unistd.h>
 #include <ctype.h>
+#include <strings.h>
 #include "bcftools.h"
 #include "filter.h"
 #include "regidx.h"

gff.c

@@ -29,6 +29,7 @@
 #include <assert.h>
 #include <inttypes.h>
 #include <string.h>
+#include <strings.h>
 #include <htslib/hts.h>
 #include <htslib/khash.h>
 #include <htslib/khash_str2int.h>

plugins/tag2tag.c

@@ -24,6 +24,7 @@ DEALINGS IN THE SOFTWARE.  */
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <strings.h>
 #include <getopt.h>
 #include <math.h>
 #include <inttypes.h>