package/bluez_utils: fix build with c99

This will fix a build failure with libsigrok

Fixes:
 - http://autobuild.buildroot.org/results/005a16e63518716c6e99c74c618d01be453c76d4



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>