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.

Modules/FindOpenSSL.cmake

@@ -143,7 +143,7 @@ if(WIN32 AND NOT CYGWIN)
     # same player, for MingW
     set(LIB_EAY_NAMES libeay32)
     set(SSL_EAY_NAMES ssleay32)
-    if(CMAKE_CROSS_COMPILING)
+    if(CMAKE_CROSSCOMPILING)
       list(APPEND LIB_EAY_NAMES crypto)
       list(APPEND SSL_EAY_NAMES ssl)
     endif()