GDB: Fix detection of ELF support when configuring with -Werror=implicit-function...

GDB: Fix detection of ELF support when configuring with -Werror=implicit-function-declaration

I am getting

    I'm sorry, Dave, I can't do that.  Symbol format `elf64-littleriscv' unknown.

errors after updating from GDB 8.3 to 10.  Bisecting showed that since
commit 1ff6de031241 ("bfd, ld: add CTF section linking"), bfd.h depends
on strncmp() being present, so configuring with
-Werror=implicit-function-declaration results in the check for ELF
support in BFD failing:

    .../gdb/gdb/../bfd/elf-bfd.h: In function 'bfd_section_is_ctf':
    .../gdb/gdb/../bfd/elf-bfd.h:3086:10: error: implicit declaration of function 'strncmp' [-Werror=implicit-function-declaration]
       return strncmp (name, ".ctf", 4) == 0 && (name[4] == 0 || name[4] == '.');

gdb/ChangeLog:

	* acincludde.m4 (GDB_AC_CHECK_BFD): Include string.h in the test
	program.

Change-Id: Iec5e21d454c2a544c44d65e23cfde552c424c18e