Build fix: filter parser fails to build with -Werror

Build fix: filter parser fails to build with -Werror

Using bison 3.8.2 and clang 15.0.7, the project fails to build with
-Werror as building the generated parser results in the following
warning:

  CXX      filter/libfilter_la-filter-parser.lo
  filter/filter-parser.cpp:1552:9: error: variable 'yynerrs' set but not used [-Werror,-Wunused-but-set-variable]
      int yynerrs = 0;
          ^

A pragma directive is inserted to silence the warning and allow the
build to go through.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I4c46a5df91bdeca6ad4e2a8ff7f2a98ee94c4f1d