linker: fix lnkr_pinned_rodata_* mismatched types

In linker-defs.h, lnkr_pinned_rodata_[start/end] are declared as
extern char. However, in linker/utils.h:linker_is_in_rodata(),
they are declared as extern const char. So remove the const in
linker_is_in_rodata() to align both declarations.

Fixes #83461

Signed-off-by: Daniel Leung <daniel.leung@intel.com>