x86: fft: elf64: fix PIC build

In a 64-bit PIC build, external functions must be called
through the PLT.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/x86/fft_mmx.asm

@@ -647,7 +647,11 @@ cglobal fft_permute, 2,7,1
     add     rsp, 8
     RET
 %elif ARCH_X86_64
+%ifdef PIC
+    jmp     memcpy wrt ..plt
+%else
     jmp     memcpy
+%endif
 %else
     push    r2
     push    r5