bfd: Change num_group to unsigned int

bfd: Change num_group to unsigned int

elf.c failed with to with GCC 10 as of

commit 906b3eb9df6c577d3f6e9c3ea5c9d7e4d1e90536
Author: Martin Liska <mliska@suse.cz>
Date:   Tue Mar 24 11:40:10 2020 +0100

    Improve endianess detection.

            PR lto/94249
            * plugin-api.h: Add more robust endianess detection.

binutils-gdb/bfd/elf.c: In function ‘setup_group’:
binutils-gdb/bfd/elf.c:740:35: error: overflow in conversion from ‘unsigned int’ to ‘int’ changes value from ‘num_group = 4294967295’ to ‘-1’ [-Werror=overflow]
  740 |     elf_tdata (abfd)->num_group = num_group = -1;
      |                                   ^~~~~~~~~
cc1: all warnings being treated as errors
make[2]: *** [Makefile:1608: elf.lo] Error 1

Change num_group in elf_obj_tdata to unsigned int to compile with GCC 10.

	PR binutils/25717
	* elf-bfd.h (elf_obj_tdata): Change num_group to unsigned int.