[CMAKE] Conditionally link "clog" in NNPack install (#15359)

"cpuinfo" is a direct dependency of NNPack. It is installed via the
NNPack's cmake script, but the version is not pinned. The "clog"
dependency was recently removed from "cpuinfo"
(pytorch/cpuinfo@3dc3103),
which leads to issues when compiling TVM with support for NNPack.

When building TVM with newly generated docker images and NNPack enabled,
we run into the following cmake configuration error:
```
The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files: NNPACK_CLOG_CONTRIB_LIB
```

This commit intends to fix this failure while also keeping compatibility
with older versions of cpuinfo. It conditionally adds "clog" to the
linker libs variable if it was found.

Author: lhutton1

cmake/modules/contrib/NNPack.cmake

@@ -32,5 +32,7 @@ if(USE_NNPACK)
   list(APPEND TVM_RUNTIME_LINKER_LIBS ${NNPACK_CONTRIB_LIB})
   list(APPEND TVM_RUNTIME_LINKER_LIBS ${NNPACK_PTHREAD_CONTRIB_LIB})
   list(APPEND TVM_RUNTIME_LINKER_LIBS ${NNPACK_CPUINFO_CONTRIB_LIB})
-  list(APPEND TVM_RUNTIME_LINKER_LIBS ${NNPACK_CLOG_CONTRIB_LIB})
+  if(NNPACK_CLOG_CONTRIB_LIB)
+    list(APPEND TVM_RUNTIME_LINKER_LIBS ${NNPACK_CLOG_CONTRIB_LIB})
+  endif(NNPACK_CLOG_CONTRIB_LIB)
 endif(USE_NNPACK)