gdb_packet: add missing <string.h> include

gdb_packet.h now calls `strlen()`, but does not have the include for it.
This causes build failures on systems where warnings are treated as
errors.

Add `#include <string.h>` to `gdb_packet.h` to fix this warning.

Signed-off-by: Sean Cross <sean@xobs.io>

src/include/gdb_packet.h

@@ -24,6 +24,7 @@
 #include <stddef.h>
 #include <stdarg.h>
 #include <stdbool.h>
+#include <string.h>
 
 /* Allow override in other platforms if needed */
 #ifndef GDB_PACKET_BUFFER_SIZE