profile: Fix the build with gcc 4.9

GCC -pedantic warns that the initialization of Header is not constant:
InstrProfilingFile.c:31:5: error: initializer element is not computable at load time [-Werror]

LLVM defaults to enabling -pedantic.  If this warning is unhelpful, we
can consider revisiting that decision.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@207784 91177308-0d34-0410-b5e6-96231b3b80d8