Fix error in Makefile

[czlitony]

This PR fixes an error in Makefile. The error was met when building mlspp with Powershell on Windows.

> make dev
# Only enable testing, not clang-tidy/sanitizers; the latter make the build
process_begin: CreateProcess(NULL, # Only enable testing, not clang-tidy/sanitizers; the latter make the build, ...) failed.
make (e=2): The system cannot find the file specified.
make: *** [Makefile:31: dev] Error 2