[cmake] Exclude iphreeqc from clang-tidy checks.

ThirdParty/iphreeqc/CMakeLists.txt

@@ -98,3 +98,6 @@ target_include_directories(iphreeqc PRIVATE
     ${CMAKE_CURRENT_SOURCE_DIR}/src/src/phreeqcpp/PhreeqcKeywords
 )
 set_target_properties(iphreeqc PROPERTIES CXX_STANDARD 14)
+# Exclude iphreeqc target from clang-tidy tests because it handles the above
+# mentioned 'src/src/Var.c' file as c, not c++.
+set_target_properties(iphreeqc PROPERTIES CXX_CLANG_TIDY "")