Fix double declaration of getauxval() for FreeBSD PPC

The extern declaration is only for Linux, move this line
into the right #ifdef section.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Co-authored-by: Martin Matuska <mm@FreeBSD.org>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #13934
Closes #13936

include/os/freebsd/spl/sys/simd_powerpc.h

@@ -53,7 +53,7 @@
 #include <sys/cdefs.h>
 
 #include <machine/pcb.h>
-#include <powerpc/cpu.h>
+#include <machine/cpu.h>
 
 #define	kfpu_allowed()		1
 #define	kfpu_initialize(tsk)	do {} while (0)
@@ -88,3 +88,5 @@ zfs_isa207_available(void)
 {
 	return ((cpu_features2 & PPC_FEATURE2_ARCH_2_07) != 0);
 }
+
+#endif