package/connman: cleanup backtrace patch

connman uses execinfo.h to dump a backtrace in case of failure.
execinfo.h is optional in uClibc, so we had a patch that conditonally
disabled backtraces for uClibc when it was missing execinfo.h

However, musl is also entirely lacking execinfo.h.

Add a proper patch that checks for execinfo.h at ./configure time. This
will no longer make any assumption on the C library that is being used.

Fixes:
    http://autobuild.buildroot.org/results/f6e/f6ee8ab3a6300f1f527f9e92b8c2ec81e63afb27/
    http://autobuild.buildroot.org/results/96b/96b78bb644ed4ef3493782521b17e3b2113a405f/


    ...

Note that there might be other issues with musl; this patch only fixes
the execinfo.h one.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>