clang-tidy: Do not warn about multiple declarations

Omit the readability-isolate-declaration [1] check that warns about
multiple declarations in one line as the s390-tools coding style prefers
this approach.

[1] https://clang.llvm.org/extra/clang-tidy/checks/readability/isolate-declaration.html

Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>

.clang-tidy

@@ -9,6 +9,7 @@ Checks: >
   -misc-include-cleaner,
   -readability-braces-around-statements,
   -readability-identifier-length,
+  -readability-isolate-declaration,
   -modernize-*,
   -performance-*,
   -portability-*,