CMakeLists: remove linking to librt

[tafilz]

This is not required anymore. Quoting glibc 2.30 NEWS:

• The functions clock_gettime, clock_getres, clock_settime,
  clock_getcpuclockid, clock_nanosleep were removed from the librt library
  for new applications (on architectures which had them). Instead, the
  definitions in libc will be used automatically, which have been available
  since glibc 2.17.

When building in a BitBake devshell as shared library and inspecting the resulting .so file with readelf reveals that librt.so is never linked to:

0x0000000000000001 (NEEDED)             Shared library: [libstdc++.so.6]
0x0000000000000001 (NEEDED)             Shared library: [libm.so.6]
0x0000000000000001 (NEEDED)             Shared library: [libgcc_s.so.1]
0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
0x0000000000000001 (NEEDED)             Shared library: [ld-linux-aarch64.so.1]
0x000000000000000e (SONAME)             Library soname: [liblibnfc_nci.so]

This will also fix

do_package_qa: QA Issue: File /usr/lib/cmake/libnfc-nci/libnfc-nci-targets.cmake in package libnfc-nci-dev contains reference to TMPDIR [buildpaths]

in yocto builds since it contained a full path to librt.so.