[indic/khmer] Remove use of global constructors

[indic/khmer] Remove use of global constructors

Alternative woul have been to resurrect F_COMBINE that I removed in
70136a78cb9eda244767f8e8a3d30d0f3c569d01

But this does it for now.  I'm not sure why check-static-inits.sh didn't
catch this before.  Clang -Weverything bot did:

  CXX      libharfbuzz_la-hb-ot-shape-complex-indic.lo
hb-ot-shape-complex-indic.cc:99:1: warning: declaration requires a global constructor [-Wglobal-constructors]
indic_features[] =
^
1 warning generated.
  CXX      libharfbuzz_la-hb-ot-shape-complex-khmer.lo
hb-ot-shape-complex-khmer.cc:36:1: warning: declaration requires a global constructor [-Wglobal-constructors]
khmer_features[] =
^
1 warning generated.