Build fix for WinCE after r148888 and r149097
https://bugs.webkit.org/show_bug.cgi?id=115168

Reviewed by Anders Carlsson.

When using compiler intrinsics on Windows CE the compiler complains
about wrong linkage specification of replaced CRT functions defined
in math.h. This is because the compiler has intrinsics for this
functions defined, and requires them to have extern "C" linkage.

* wtf/MathExtras.h:
(wtf_atan2): Added extern "C" to function signature.
(wtf_fmod): Ditto.
(wtf_pow): Ditto.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149285 268f45cc-cd09-0410-ab3c-d52691b4dbfc