Fix gitversion array size to include null terminator

[edsammy]

Fixes build error with newer GCC versions that enforce null terminator space in string arrays.

Matches upstream fix from commaai#2309.

[heysenbug]

Which GCC version is this a fix for?

[edsammy]

From downquark7

Including space for the null terminator in this string fixes a warning that was not added to gcc until version 15. Compile code with gcc version 15 to reproduce this error.

After running setup.sh arm-none-eabi-gcc got upgraded to 15.2.1 20251203 and was causing the following error on board builds

In file included from board/bootstub.c:18:
./board/obj/gitversion.h:2:32: error: initializer-string for array of 'unsigned char' truncates NUL terminator but destination lacks 'nonstring' attribute (19 chars into 18 available) [-Werror=unterminated-string-initialization]
    2 | const uint8_t gitversion[18] = "DEV-395acfb9-DEBUG";
      |                                ^~~~~~~~~~~~~~~~~~~~
In file included from board/bootstub.c:18:
./board/obj/gitversion.h:2:32: error: initializer-string for array of 'unsigned char' truncates NUL terminator but destination lacks 'nonstring' attribute (19 chars into 18 available) [-Werror=unterminated-string-initialization]
    2 | const uint8_t gitversion[18] = "DEV-395acfb9-DEBUG";
      |                                ^~~~~~~~~~~~~~~~~~~~
In file included from board/jungle/main.c:20:
./board/obj/gitversion.h:2:32: error: initializer-string for array of 'unsigned char' truncates NUL terminator but destination lacks 'nonstring' attribute (19 chars into 18 available) [-Werror=unterminated-string-initialization]
    2 | const uint8_t gitversion[18] = "DEV-395acfb9-DEBUG";
      |                                ^~~~~~~~~~~~~~~~~~~~
In file included from board/main.c:23:
./board/obj/gitversion.h:2:32: error: initializer-string for array of 'unsigned char' truncates NUL terminator but destination lacks 'nonstring' attribute (19 chars into 18 available) [-Werror=unterminated-string-initialization]
    2 | const uint8_t gitversion[18] = "DEV-395acfb9-DEBUG";
    ```

[heysenbug]

Addressed in #8