Avoid installing kernel headers on FreeBSD

The kernel headers are installed for DKMS on linux, so don't install
them unless we're building on linux.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10506

include/sys/sysevent/Makefile.am

@@ -8,6 +8,8 @@ libzfs_HEADERS = $(COMMON_H)
 endif
 
 if CONFIG_KERNEL
+if BUILD_LINUX
 kerneldir = @prefix@/src/zfs-$(VERSION)/include/sys/sysevent
 kernel_HEADERS = $(COMMON_H)
 endif
+endif