Disable -Wmisleading-indentation

This was introduced recently to Clang and it fails for:

  glslang/MachineIndependent/Scan.cpp:197:17: error: misleading indentation; statement is not part of the previous 'else'

Change-Id: I767ee11c31bf80c81cedda20880cd8947de545c6
diff --git a/BUILD.gn b/BUILD.gn
index 5d5b150..7dbff8d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -144,6 +144,8 @@
       "-Wno-implicit-fallthrough",
       "-Wno-ignored-qualifiers",
       "-Wno-unused-variable",
+      "-Wno-unknown-warning-option",
+      "-Wno-misleading-indentation",
     ]
   }