package/libsndfile: fix c99 check

package/libsndfile: fix c99 check

Since the bump of libsndfile to version 1.0.31 in commit
8c600fda64f48c16d20588336dd7ac6a32241c6a, the check for c99 contains
"include <wchar.h>" although the package itself does not use wchar
functions outside Windows-specific code.  This patch forces the
detection of c99 for toolchains lacking wchar.

Fixes:
http://autobuild.buildroot.net/results/62c/62c07e66491e2fcf32d3921f8286ec1293591194/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>