gdbstub: Fix qOffsets packet detection

qOffsets has no additional optional parameters. So match the complete
string to avoid stumbling over possible future commands with identical
prefix.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

gdbstub.c

@@ -1107,7 +1107,7 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
             break;
         }
 #ifdef CONFIG_USER_ONLY
-        else if (strncmp(p, "Offsets", 7) == 0) {
+        else if (strcmp(p, "Offsets") == 0) {
             TaskState *ts = s->c_cpu->opaque;
 
             snprintf(buf, sizeof(buf),