package/exiv2: disable on musl

package/exiv2: disable on musl

Musl build is broken:

/home/buildroot/build/instance-0/output/build/exiv2-910f3507795e1930ae216c9febee0bf9a88e99c0/src/actions.cpp:2052:32:
 error: 'PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP' was not declared in this scope

According to http://www.openwall.com/lists/musl/2017/02/20/3
the define is not available on musl.

Fixes
http://autobuild.buildroot.net/results/283/28334a8a0b23c7c8260023b9a3aa15e8dd7613d0/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>