pkg-cmake: change sed expression delimiter

Switch from : to # since CFLAGS can include :'s spilled in from
BR2_TARGET_OPTIMIZATION, for example:

BR2_TARGET_OPTIMIZATION="-Wl,-rpath,/lib:/usr/lib" would cause the sed
expression to fail thus breaking the build.

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>