gh-90815: Fix _PyMem_MimallocEnabled() compiler warning

[vstinner]

Don't declare _PyMem_MimallocEnabled() if WITH_PYMALLOC macro is not defined (./configure --without-pymalloc).

Fix also a typo in _PyInterpreterState_FinalizeAllocatedBlocks().

• Issue: Add mimalloc memory allocator #90815

[vstinner]

I tested that no warnings is emitted with these 4 configurations:

pyconfigure_debug --with-pymalloc --with-mimalloc && make clean && make pyconfigure_debug --with-pymalloc --without-mimalloc && make clean && make pyconfigure_debug --without-pymalloc --with-mimalloc && make clean && make pyconfigure_debug --without-pymalloc --without-mimalloc && make clean && make 

with:

$ alias pyconfigure_debug alias pyconfigure_debug='./configure --cache-file=../python-config.cache --with-pydebug CFLAGS=-O0 --with-system-expat'