FindOpenSSL: Fix spelling of CMAKE_CROSSCOMPILING (#14075)

Use of the mis-spelled variable name 'CMAKE_CROSS_COMPILING' was
introduced by commit 4b793ad1 (FindOpenSSL: find cross-compiled OpenSSL
from MinGW, 2012-07-23).  Fix the spelling.
Merge topic 'FindOpenSSL-cross-compiling' into next

1b614e0 FindOpenSSL: Fix spelling of CMAKE_CROSSCOMPILING (#14075)