Adjust gitversion handling to include null terminator in length calculations.

[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.

In file included from panda/board/bootstub.c:18:
panda/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-5f3c09c9-DEBUG";
      |                                ^~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
scons: *** [panda/board/board/obj/body_h7bootstub.o] Error 1
scons: building terminated because of errors.