testsuite: gcc.dg/pr106397.c: Add -w to options

Targets that don't support prefetching get a warning:
cc1: warning: '-fprefetch-loop-arrays' not supported for this target

Align with other tests passing -fprefetch-loop-arrays for
all targets: add "-w" to options.

	* gcc.dg/pr106397.c: Add -w to options.