text-utils/hexsyntax.c: fix for missing program_invocation_short_name

text-utils/hexsyntax.c: fix for missing program_invocation_short_name

Some libc do not expose program_invocation_short_name. Therefore util-linux
comes with helpers in 'c.h'. Use the 'c.h' in hexsyntax.c where it was missed.

This patch fixes following error for me:
---8<---
hexsyntax.c: In function 'newsyntax':
hexsyntax.c:115: error: 'program_invocation_short_name' undeclared (first use in this function)
hexsyntax.c:115: error: (Each undeclared identifier is reported only once
hexsyntax.c:115: error: for each function it appears in.)
--->8---

Signed-off-by: Andreas Bießmann <biessmann@corscience.de>