package/minizip: use pkg-config to find zstd

Fix build with zstd in version 2.10.0

Fixes:
 - http://autobuild.buildroot.org/results/0fe0ca0a7d39f863614f2aecd9eefc20cd64daf4



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