textproc/libxml2: Fix cmake file

The port's cmake file is wrong because there is an unclosed
if-statement.
Can check with 'cmake -P test-libxml2.cmake' where the content of
test-libxml2.cmake is:
include("/usr/local/lib/cmake/libxml2/libxml2-config.cmake")

The test's result is:
CMake Error at /usr/local/lib/cmake/libxml2/libxml2-config.cmake:53 (if):

Flow control statements are not properly nested.

Call Stack (most recent call first):

test-libxml2.cmake:1 (include)

Approved by: portmgr (blanket, trivial runtime fix)