tensor-filter-common: unnecessary usage of volatile

tensor-filter-common: unnecessary usage of volatile

GLib-Once (g_once_init_enter/leave) have already taken
care of concurrency.
Its users do not need to worry about cache coherency or
concurrency issues for the initialization.

Actually, as #3440 is showing, the volatile keyword
is neglected by g_once_init_enter function, making
the usage of volatile meaningless anyway.

Remove volatile keyword and mitigate the first
compiler warning-error of #3440

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>