Fixes warning from gcc 13 that is becoming an error in gcc 14

imake.c: In function ‘doit’:
imake.c:797:29: error: passing argument 2 of ‘execvp’ from incompatible
 pointer type [-Werror=incompatible-pointer-types]
  797 |                 execvp(cmd, argv);
      |                             ^~~~
      |                             |
      |                             const char **
In file included from imake.c:172:
/usr/include/unistd.h:359:33: note: expected ‘char * const*’
 but argument is of type ‘const char **’
  359 | extern int execvp(const char *, char *const *);
      |                                 ^~~~~~~~~~~~~

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>