Fix extern warnings produced by ittnotify_static.c

when compiling with gcc or clang numerous warnings concerning the usage 
of extern "C" linkage.  All the __kmp_itt_sync* variables are declared
like: extern "C" type __kmp_itt_sync... = definition; through various macros.
This note from cppreference.com explains why this is a problem.
// From http://en.cppreference.com/w/cpp/language/language_linkage
extern "C" int x; // a declaration and not a definition
// The above line is equivalent to extern "C" { extern int x; }
extern "C" { int x; } // a declaration and definition
Since the __kmp_itt_* variables are being declared and defined, these variables 
should use the bracketed version instead.

llvm-svn: 239184