timidity: fix build with gcc15

- add "-std=gnu17" to `env.NIX_CFLAGS_COMPILE`

Upstream had no version updates since 2018
Other distros also opted for "-std=gnu11" or "-std=gnu17":
https://src.fedoraproject.org/rpms/timidity++/c/ce4c5633a1d10d9065a0da748e27308a7c5c4bc7
https://github.com/gentoo/gentoo/commit/80744c4db244cadc7a5bf9da9d975404222431d6

Fixes build failure with gcc15:
```
nkflib.c:389:5: error: conflicting types for 'line_fold'; have 'int(void)'
  389 | int line_fold();
      |     ^~~~~~~~~
nkflib.c:319:17: note: previous declaration of 'line_fold' with
type 'int(int,  int)'
  319 | static  int     line_fold(int c2,int c1);
      |                 ^~~~~~~~~
nkflib.c: In function 'e_oconv':
nkflib.c:1594:16: error: too many arguments to function 'line_fold';
expected 0, have 2
 1594 |         switch(line_fold(c2,c1)) {
      |                ^~~~~~~~~ ~~
nkflib.c:389:5: note: declared here
  389 | int line_fold();
      |     ^~~~~~~~~
nkflib.c: In function 's_oconv':
nkflib.c:1646:16: error: too many arguments to function 'line_fold';
expected 0, have 2
 1646 |         switch(line_fold(c2,c1)) {
      |                ^~~~~~~~~ ~~
nkflib.c:389:5: note: declared here
  389 | int line_fold();
      |     ^~~~~~~~~
nkflib.c: In function 'j_oconv':
nkflib.c:1694:16: error: too many arguments to function 'line_fold';
expected 0, have 2
 1694 |         switch(line_fold(c2,c1)) {
      |                ^~~~~~~~~ ~~
nkflib.c:389:5: note: declared here
  389 | int line_fold();
      |     ^~~~~~~~~
nkflib.c: At top level:
nkflib.c:1829:1: error: conflicting types for 'line_fold';
have 'int(int,  int)'
 1829 | line_fold(int c2, int c1)
      | ^~~~~~~~~
nkflib.c:389:5: note: previous declaration of 'line_fold' with
type 'int(void)'
  389 | int line_fold();
      |     ^~~~~~~~~
```