science/InsightToolkit: Modify patch to use ${CMAKE_DL_LIBS}

${CMAKE_DL_LIBS} expands to "" on FreeBSD where the dl functions are
included in the Standard C library (libc), but to "dl" on operating
systems where the dl functions are part of libdl.

This change to the patch does not result in a change in contents of the
resulting package.

Reported by: jbeich