powerpc: math-emu: remove unneeded header search paths

The header search path -I. in kernel Makefiles is very suspicious;
it allows the compiler to search for headers in the top of $(srctree),
where obviously no header file exists.

-Iinclude/math-emu seems unnecessary because all files include headers
in the form of #include <math-emu/...>.

I was able to build without these header search paths.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>