clang-tidy: suppress new readability-identifier-length lint

CMake uses short names all over the place; 3 character minimums is
excessive.

.clang-tidy

@@ -24,6 +24,7 @@ readability-*,\
 -readability-convert-member-functions-to-static,\
 -readability-function-cognitive-complexity,\
 -readability-function-size,\
+-readability-identifier-length,\
 -readability-identifier-naming,\
 -readability-implicit-bool-conversion,\
 -readability-inconsistent-declaration-parameter-name,\