c64-debugger: fix the build against gcc-15

Without the chnage the build fails on `master` as
https://hydra.nixos.org/build/324451165:

```
In file included from Games/c64/Emulators/vice/monitor/asm6502.c:38:
Games/c64/Emulators/vice/monitor/montypes.h:44:13: error: 'bool' cannot be defined via 'typedef'
   44 | typedef int bool;
      |             ^~~~
Games/c64/Emulators/vice/monitor/montypes.h:44:13: note: 'bool' is a keyword with '-std=c23' onwards
```

pkgs/by-name/c6/c64-debugger/package.nix

@@ -46,7 +46,7 @@ stdenv.mkDerivation {
 
     # Build C64 debugger
     make -C MTEngine \
-      CFLAGS="-w -O2 -fcommon" \
+      CFLAGS="-w -O2 -fcommon -std=gnu17" \
       CXXFLAGS="-w -O2 --std=c++11" \
       DEFINES="-DRUN_COMMODORE64" \
       -j$NIX_BUILD_CORES
@@ -55,7 +55,7 @@ stdenv.mkDerivation {
 
     # Build 65XE debugger
     make -C MTEngine \
-      CFLAGS="-w -O2 -fcommon" \
+      CFLAGS="-w -O2 -fcommon -std=gnu17" \
       CXXFLAGS="-w -O2 --std=c++11" \
       DEFINES="-DRUN_ATARI" \
       -j$NIX_BUILD_CORES
@@ -64,7 +64,7 @@ stdenv.mkDerivation {
 
     # Build NES debugger
     make -C MTEngine \
-      CFLAGS="-w -O2 -fcommon" \
+      CFLAGS="-w -O2 -fcommon -std=gnu17" \
       CXXFLAGS="-w -O2 --std=c++11" \
       DEFINES="-DRUN_NES" \
       -j$NIX_BUILD_CORES