cmake: ld: add application source dir to linker script cpp command

To enable using application-defined sections in linker scripts, the
application source directory must be included in the search path when
running the preprocessor to generate linker scripts.

This prevents build failures when using any of the configuration options
CONFIG_CUSTOM_RODATA_LD, CONFIG_CUSTOM_RWDATA_LD and
CONFIG_CUSTOM_SECTIONS_LD.

Signed-off-by: Kristian Klomsten Skordal <kristian.skordal@nordicsemi.no>