common: splash_source: Don't compile ubifs splash cmd for SPL build

Don't compile ubifs splash mount command for SPL build as this command
is not supported (and also not enabled) during SPL stage.

This fixes below compilation error, when CONFIG_CMD_UBIFS is enabled in
defconfig along with splash screen enabled at SPL stage:

"aarch64-none-linux-gnu-ld.bfd: common/cli.o: in function `run_command':
u-boot-next/u-boot/common/cli.c:51: undefined reference to
`parse_string_outer'
u-boot-next/u-boot/common/cli.c:51:(.text.run_command+0x10): relocation
truncated to fit: R_AARCH64_JUMP26 against undefined symbol
`parse_string_outer' make[2]: ***
[u-boot-next/u-boot/scripts/Makefile.spl:527: spl/u-boot-spl] Error 1
make[1]: *** [u-boot-next/u-boot/Makefile:2053: spl/u-boot-spl] Error 2
make[1]: Leaving directory 'u-boot-next/u-boot/out/a53' make: ***
[Makefile:177: sub-make] Error 2"

Fixes: eb9217dc ("common: Enable splash functions at SPL")
Signed-off-by: Devarsh Thakkar <devarsht@ti.com>
Reviewed-by: Nikhil M Jain <n-jain1@ti.com>