cmake: exclude .clang-format from opae headers

[pcolberg]

This resolves an rpmbuild failure when building packages from a source tarball created with git archive.

RPM build errors:
    Installed (but unpackaged) file(s) found:
   /usr/include/opae/cxx/.clang-format

[coveralls]

Pull Request Test Coverage Report for Build 7981357790

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 64.658%

---

Totals
Change from base Build 7921222968:	0.0%
Covered Lines:	15809
Relevant Lines:	24450

---

💛 - Coveralls