libclc: Add misssing quotes around target list

libclc: Add misssing quotes around target list

Default for LIBCLC_TARGETS is `all` so a single
target is ok but when its overidden with a semi-colon
separated list to use a subset of targets, e.g.

LIBCLC_TARGETS:class-nativesdk = "amdgcn--;amdgcn--amdhsa;nvptx--;nvptx64--;nvptx--nvidiacl;nvptx64--nvidiacl"

cmake rejects it.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>