clang-format: add IndentExternBlock setting

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I7cb11f19333775f3912f98a3a8bfd6559601fabc
diff --git a/style/c/.clang-format b/style/c/.clang-format
index 6da79a0..0683c55 100644
--- a/style/c/.clang-format
+++ b/style/c/.clang-format
@@ -67,6 +67,7 @@
     Priority: 1
 IncludeIsMainRegex: '(Test)?$'
 IndentCaseLabels: false
+IndentExternBlock: NoIndent
 #IndentPPDirectives: None # Unknown to clang-format-5.0
 IndentWidth: 8
 IndentWrappedFunctionNames: false