gdb_packet: add missing <string.h> include

[xobs]

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.

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[dragonmux]

Ah, good catch - LGTM, merging. Thank you for the contribution!