Makefile.libcompat: Fix DTRACE variable

Back when Makefile.lib32 was turned into Makefile.libcompat, a typo was
introduced; it should have been LIBCOMPATDTRACE, but instead gained a
dollar sign in it as LIB$COMPATDTRACE (and with COMPATDTRACE not being
defined, was equivalent to ${LIB:U${DTRACE}}). This then meant it was
not converted to LIB${_LIBCOMPAT}DTRACE in 91d7edd549f5 ("Generalise
libcompat to be a list rather than a single option") and instead left
broken. Belatedly fix this long-standing seemingly-inconsequential bug.

Fixes: dba75e9fa5a9 ("Move Makefile.lib32 to Makefile.libcompat and generalize it.")