fix warnings when compiling with gcc8

E.g. when compiling with gcc8 with warnings as errors you'll get
the following:

mode_s.c: In function 'nav_modes_to_string':
mode_s.c:1626:9: error: 'strncat' specified bound 10 equals source length [-Werror=stringop-overflow=]
         strncat(buf, "autopilot ", 10);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This error is indicating that you should be using strcat instead of strncat
since the length of source is same as the bound.

mode_s.c

@@ -1616,17 +1616,17 @@ static const char *nav_modes_to_string(nav_modes_t flags) {
 
  buf[0] = 0;
  if (flags & NAV_MODE_AUTOPILOT)
- strncat(buf, "autopilot ", 10);
+ strcat(buf, "autopilot ");
  if (flags & NAV_MODE_VNAV)
- strncat(buf, "vnav ", 6);
+ strcat(buf, "vnav ");
  if (flags & NAV_MODE_ALT_HOLD)
- strncat(buf, "althold ", 8);
+ strcat(buf, "althold ");
  if (flags & NAV_MODE_APPROACH)
- strncat(buf, "approach ", 9);
+ strcat(buf, "approach ");
  if (flags & NAV_MODE_LNAV)
- strncat(buf, "lnav ", 5);
+ strcat(buf, "lnav ");
  if (flags & NAV_MODE_TCAS)
- strncat(buf, "tcas ", 5);
+ strcat(buf, "tcas ");
 
  if (buf[0] != 0)
  buf[strlen(buf) - 1] = 0;