kallsyms: fix build without execinfo

Some libc's like musl libc don't provide execinfo.h since it's not part of
POSIX.  In order to fix compilation on musl, only include execinfo.h if
available (HAVE_BACKTRACE_SUPPORT)

This was discovered with c104c160 ("Kunit to check the longest symbol
length") which starts to include linux/kallsyms.h with Alpine Linux'
configs.

Link: https://lkml.kernel.org/r/20250622014608.448718-1-fossdd@pwned.life


Fixes: c104c160 ("Kunit to check the longest symbol length")
Signed-off-by: Achill Gilgenast <fossdd@pwned.life>
Cc: Luis Henriques <luis@igalia.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>