mooshimeter-dmm: silence compiler warning (memset() prototypes)

The Mooshimeter driver uses mem*() and str*() library calls. Include the
<string.h> header file to silence compiler warnings.

  ...
  ./src/hardware/mooshimeter-dmm/protocol.c: In function 'lookup_tree_path':
  ./src/hardware/mooshimeter-dmm/protocol.c:275:3: warning: implicit declaration of function 'strchr' [-Wimplicit-function-declaration]
     end = strchr(path, ':');
     ^
  ./src/hardware/mooshimeter-dmm/protocol.c:275:9: warning: incompatible implicit declaration of built-in function 'strchr' [enabled by default]
     end = strchr(path, ':');
           ^
  ...

src/hardware/mooshimeter-dmm/protocol.c

@@ -20,6 +20,7 @@
 #include <config.h>
 #include <gio/gio.h>
 #include <math.h>
+#include <string.h>
 #include "protocol.h"
 
 /*