Add missing operand which clang complains about but GCC assumes it to be
'm1' if not specified.
Works around build failure with Clang:
| src/libswscale/riscv/rgb2rgb_rvv.S:88:25: error: operand must be e[8|16|32|64|128|256|512|1024],m[1|2|4|8|f2|f4|f8],[ta|tu],[ma|mu]
| vsetvli t4, t3, e8, ta, ma
| ^
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>