shared-qmi: include string.h for memset()

This patch fixes the following compiler warning:

mm-shared-qmi.c:447:9: error: implicitly declaring library function 'memset' with type 'void *(void *, int, unsigned long)' [-Werror,-Wimplicit-function-declaration]
        memset (buf, 0, sizeof (buf));
        ^

src/mm-shared-qmi.c

@@ -14,6 +14,7 @@
  */
 
 #include <config.h>
+#include <string.h>
 #include <arpa/inet.h>
 
 #include <glib-object.h>