newlib: Fix typo in Kconfig related to NEWLIB_LIBC_ALIGNED_HEAP_SIZE

When we introduced NEWLIB_LIBC_ALIGNED_HEAP_SIZE in commit
42a2c964.  We accidently had the Kconfig
symbol depend on CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT the leading
'CONFIG_' shouldn't exist.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>