stacktrace: Init elf_fd in sysprof_init_dwfl

stacktrace: Init elf_fd in sysprof_init_dwfl

When building with LTO gcc believes elf_fd can be used uninitialized:

In function ‘sysprof_init_dwfl’,
    inlined from ‘sysprof_unwind_cb’ at stacktrace.c:1235:16:
stacktrace.c:1087:7: error: ‘elf_fd’ may be used uninitialized [-Werror=maybe-uninitialized]
 1087 |       close (elf_fd);
      |       ^

This code won't be reached because if find_procfile doesn't initialize
elf_fd, it will return an error. But help the compiler by initializing
elf_fd to -1.

	* src/stacktrace.c (sysprof_init_dwfl): Init elf_fd to -1.

Signed-off-by: Mark Wielaard <mark@klomp.org>