libctf: add missing header in BFD ELF check

libctf: add missing header in BFD ELF check

Without this, GCC warns:

In file included from conftest.c:36:
../../libctf/../bfd/elf-bfd.h: In function 'bfd_section_is_ctf':
../../libctf/../bfd/elf-bfd.h:3089:10: warning: implicit declaration of function 'strncmp' [-Wimplicit-function-declaration]
 3089 |   return strncmp (name, ".ctf", 4) == 0 && (name[4] == 0 || name[4] == '.');
      |          ^~~~~~~
../../libctf/../bfd/elf-bfd.h:3089:33: warning: 'strncmp' argument 3 type is 'int' where 'long unsigned int' is expected in a call to built-in function declared without prototype [-Wbuiltin-declaration-mismatch]
 3089 |   return strncmp (name, ".ctf", 4) == 0 && (name[4] == 0 || name[4] == '.');
      |                                 ^
<built-in>: note: built-in 'strncmp' declared here

These warnings do not currently throw off the result of the configure
check, but it's better to squash them anyway.

libctf/ChangeLog
2021-02-03  Nick Alcock  <nick.alcock@oracle.com>

	* configure.ac (ac_cv_libctf_bfd_elf): Include string.h.
	* configure: Regenerated.