[NFC][cmake] Build fix in tools/llvm-config/CMakeLists.txt

To avoid the following error message (using cmake version 3.13.4) :

```
CMake Error at tools/llvm-config/CMakeLists.txt:37 (string):
Syntax error in cmake code  when parsing string-std=[^ ]\+Invalid escape sequence \+
```

Reviewed By: mgorny

Differential Revision: https://reviews.llvm.org/D58619

llvm-svn: 369887