arm64: mm: Export __flush_icache_range() to modules

lkdtm calls flush_icache_range(), which results in an out-of-line call
to __flush_icache_range(), which is not exported to modules.

Export the symbol to modules to fix this build breakage.

Fixes: 3b8c9f1c

 ("arm64: IPI each CPU after invalidating the I-cache for kernel mappings")
Signed-off-by: Will Deacon <will.deacon@arm.com>