PR 25611, PR 25614: GAS: Remove a double inclusion of "bignum.h"

PR 25611, PR 25614: GAS: Remove a double inclusion of "bignum.h"

Correct an issue with commit 5496f3c635dc ("Add support for generating
DWARF-5 format directory and file name tables from the assembler.") and
remove a duplicate direct inclusion of "bignum.h" from dwarf2dbg.c that
causes a GAS compilation error:

In file included from .../gas/dwarf2dbg.c:33:
.../gas/bignum.h:42: error: redefinition of typedef 'LITTLENUM_TYPE'
.../gas/bignum.h:42: error: previous declaration of 'LITTLENUM_TYPE' was here
make[4]: *** [dwarf2dbg.o] Error 1

with some GCC versions, as this header has been already included via
"as.h" and then "flonum.h".

	gas/
	PR 25611
	PR 25614
	* dwarf2dbg.c: Do not include "bignum.h".