package/git: use pkg-config to get ssl dependencies

package/git: use pkg-config to get ssl dependencies

On some architectures, atomic binutils are provided by the libatomic
library from gcc. Linking with libatomic is therefore necessary,
otherwise the build fails with:

/home/test/autobuild/run/instance-2/output/host/sparc-buildroot-linux-uclibc/sysroot/usr/lib/libssl.a(ssl_cert.o): In function `CRYPTO_DOWN_REF':
/home/test/autobuild/run/instance-2/output/build/libopenssl-1.1.1a/include/internal/refcount.h:50: undefined reference to `__atomic_fetch_sub_4'

This is often for example the case on sparcv8 32 bit.

To fix this issue, use pkg-config to retrieve openssl dependencies
including atomic library, these dependencies must be passed to
LIB_4_CRYPTO IN GIT_MAKE_OPTS

Fixes:
 - http://autobuild.buildroot.org/results/3093897d14a854a7252b25b2fa1f8fdcbb26c9b7

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>