Fix -Wincompatible-pointer-types warning (Issue #3)

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 *);
      |                                 ^~~~~~~~~~~~~

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merged