l1if: Include string.h to fix possible coverity issue

Coverity warns about the below lines of code. strerror is supposed
to return a char* and %s should work on char. Let's see if including
string.h will make coverity more happy. The pre-processes file looked
sane as well.

 LOGP(DL1IF, LOGL_ERROR, "error writing to L1 msg_queue: %s\n",
               strerror(errno));

Coverity: CID 1040951, CID 1040950, CID 1040952

src/sysmo_l1_hw.c

@@ -24,6 +24,7 @@
 #include <unistd.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <string.h>
 
 #include <sys/types.h>
 #include <sys/stat.h>