tests: kernel: fp_sharing: minor fix in #ifdef expression

We need to use the ARMV7_M_ARMV8_M_FP Kconfig symbol,
which denotes the Floating-Point capabilities, instead
of the option that signifies the Cortex-M variant. Fix
is of minor importance, as long as the #ifdef block
remains empty.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>