Clang requires the __getReg function to be declared in addition to
be declared as an intrinsic with the pragma.

This fixes the following error:

../wine/include/winnt.h:2412:27: error: call to undeclared library function '__getReg' with type 'unsigned long long (int)'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    return (struct _TEB *)__getReg(18);
                          ^
../wine/include/winnt.h:2412:27: note: include the header <intrin.h> or explicitly provide a declaration for '__getReg'
1 error generated.
Signed-off-by: Martin Storsjö <martin@martin.st>