gallium: fix broken if-macro for PIPE_ARCH_LITTLE_ENDIAN
ae071434e is a commit that was backported from master in 19.2. That commit also copied logic from f9f60da81, which causes a compilation failure on ppc64le because the macro itself was not changed as well.
Change the #if
to an #ifdef
to fix this issue.
Fixes #2131 (closed)