[3.13] gh-116984: Make mimalloc header includes relative to the current file (GH-118808) (#118866)

Some embedders and extensions include parts of the internal API. The
pycore_mimalloc.h file is transitively include by a number of other
internal headers. This avoids include errors for code that was
already including those headers.
(cherry picked from commit 71cc0651e79041abd648595f3030dfa41009137a)

Co-authored-by: Sam Gross <colesbury@gmail.com>