coregrind/m_gdbserver/remote-utils.c (prepare_resume_reply): Use memcpy

coregrind/m_gdbserver/remote-utils.c (prepare_resume_reply): Use memcpy

GCC8 (but apparently not later versions) complain about the use of
strncpy when not actually copying a string:

remote-utils.c:1140:14: warning: 'char* strncpy(char*, const char*, size_t)' output truncated before terminating nul copying 6 bytes from a string of the same length [-Wstringop-truncation]
          strncpy (buf, "watch:", 6);
          ~~~~~~~~^~~~~~~~~~~~~~~~~~

This is "harmless" because buf is large enough and we will add more
chars (including a zero terminator) later. But using strncpy here is a
bit odd because we don't really want to copy a string, but an array of
6 chars. So use memcpy here to do so, simplyfing the code.