Fix Zstd kernel build with GCC 4.2

By disabling the -Winline warning. Fixes the powerpc and sparc64 build
after r327706.

Note: MIPS and RISCV builds still broken due to absense of ctzdi2 (aka
builtin_ctzll) in their libgcc or libcompiler-rt libraries.

Reported by: markj
Sponsored by: Dell EMC Isilon