spirv-tools: Correctly set the prefix in exported cmake packages

spirv-tools: Correctly set the prefix in exported cmake packages

spirv-tools exports Cmake packages.  When they are installed the
INTERFACE_LINK_LIBRARIES variable is not correctly updated to use
_IMPORT_PREFIX like the other variables. This may have something to do with
the path to the sysroot being different than the source path.

The existing recipe recognizes that the sysroot path made it through to the
install and tries to get rid of it, but this just ends up with dependent
tools looking in vain for /usr/lib/librt.so.

Replace the INTERFACE_LINK_LIBRARIES value in SPIRV-ToolsTarget.cmake with
"${IMPORT_PREFIX}/lib".

Signed-off-by: Jordan Crouse <jorcrous@amazon.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>