package/harfbuzz: fix build without threads

Fix the following build failures without threads by defining HB_NO_MT as
specified in https://github.com/harfbuzz/harfbuzz/blob/main/CONFIG.md:

In file included from ../src/hb.hh:458,
                 from ../src/hb-blob.cc:28:
../src/hb-mutex.hh:88:2: error: #error "Could not find any system to define mutex macros."
   88 | #error "Could not find any system to define mutex macros."
      |  ^~~~~

This build failure is raised since bump to version 2.8.1 in commit
e9ba1c80 and
https://github.com/harfbuzz/harfbuzz/commit/711c241f6c7e18c5403602375a733af74df76f83
which removed busyloop mutex implementation.

Fixes:
 - http://autobuild.buildroot.org/results/0c03aca5e3ade735b54b0b9233896b868aec1520



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>