Fix -Wunterminated-string-initialization warning in ulong_to_str

[cxzhong]

Replace string literal with explicit char array initializer to avoid the -Wunterminated-string-initialization warning from GCC15.
fix

[8/15] Compiling C object src/cys...ated_src_cysignals_signals.pyx.c.o
In file included from src/cysignals/signals.cpython-314-x86_64-linux-gnu.so.p/src/cysignals/signals.pyx.c:1133:
../../src/cysignals/implementation.c: In function ‘ulong_to_str’:
../../src/cysignals/implementation.c:163:30: warning: initializer-string for array of ‘char’ truncates NUL terminator but destination lacks ‘nonstring’ attribute (17 chars into 16 available) [-Wunterminated-string-initialization]
  163 |     const char xdigits[16] = "0123456789abcdef";
      |                              ^~~~~~~~~~~~~~~~~~