package/rtl_433: force Release build

Force Release build to avoid the following build failure which is raised
since the addition of the package in commit
4634e849:

/tmp/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/i586-buildroot-linux-musl/9.3.0/../../../../i586-buildroot-linux-musl/bin/ld: cannot find -lasan
/tmp/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/i586-buildroot-linux-musl/9.3.0/../../../../i586-buildroot-linux-musl/bin/ld: cannot find -lubsan

Fixes:
 - http://autobuild.buildroot.org/results/83274082879f9599bc106e12a253f4b9229ea00b



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>