x86: ensure invalidate_icache() definition is visible only when !__ASSEMBLY__

x86: ensure invalidate_icache() definition is visible only when !__ASSEMBLY__

Commit edff605421 introduces an empty invalidate_icache() function in
page.h for x86 but mistakenly places it outside the !__ASSEMBLY__
block. This causes build failure on x86.

Address this by moving the function definition to within the existing
!__ASSEMBLY__ block.

Fixes: edff605421 ("Avoid excess icache flushes in populate_physmap() before domain has been created")
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>