rtcd: relocate 'extern "C" {' declaration

rtcd: relocate 'extern "C" {' declaration

Place after pulling in forward declarations from the codec. This fixes
compilation of the tests under vs9.

Since
10783d4 Adds high bitdepth transform functions and tests
where vp9_idct.h was added to vp9_rtcd.h the tests are pulling in
vp9_systemdependent.h, which under visual studio include intrin.h. With
VS9 these include headers which define helper classes for intel
intrinsics. When including it in the tests (via vp9_rtcd.h) __cplusplus
is defined but vp9_rtcd.h would wrap declarations in 'extern "C" {'
causing a mismatch in linkage which resulted in compilation failure.

Change-Id: I475e50198b71320e8606bc95c9454876d8799ede