Fix failing building with custom libzip

[mannreis]

I've bumped into a problem when building netcdf's nczarr target because of zip.h not found, although CMake was resolving the dependencies correctly:

-- Found Zip: headers at /sw/spack-levante/libzip-1.2.0-qjdhu5/include, libraries at /sw/spack-levante/libzip-1.2.0-qjdhu5/lib
-- 	library is /sw/spack-levante/libzip-1.2.0-qjdhu5/lib/libzip.so

$ cmake -L  2> /dev/null| grep -i libzip
Zip_INCLUDE_DIRS:PATH=/sw/spack-levante/libzip-1.2.0-qjdhu5/include
Zip_RELEASE_LIBRARY:FILEPATH=/sw/spack-levante/libzip-1.2.0-qjdhu5/lib/libzip.so
$ cmake --build . --target nczarr
[  0%] Building C object libnczarr/CMakeFiles/nczarr.dir/zmap_zip.c.o
/home/k/k202186/repos/netcdf-c/libnczarr/zmap_zip.c:15:10: fatal error: zip.h: No such file or directory
 #include <zip.h>
          ^~~~~~~
compilation terminated.
gmake[3]: *** [libnczarr/CMakeFiles/nczarr.dir/build.make:412: libnczarr/CMakeFiles/nczarr.dir/zmap_zip.c.o] Error 1
gmake[2]: *** [CMakeFiles/Makefile2:2466: libnczarr/CMakeFiles/nczarr.dir/all] Error 2
gmake[1]: *** [CMakeFiles/Makefile2:2473: libnczarr/CMakeFiles/nczarr.dir/rule] Error 2
gmake: *** [Makefile:784: nczarr] Error 2

This doesn't happen on my local build (with std/sudo locations for libzip). But on a HPC environment I'm relying on a particular libzip module and this seems to fix my problem