build: remove bad double negative when discovering 'git'

This automatically works because CMake's `if` statement considers
variables which' content ends with "-NOTFOUND" to be FALSE.

The previous version however was first tried as a variable which didn't
exist. As a result after that it's interpreted as a string (not a
variable anymore). So it skips the "-NOTFOUND" check and only does a
"string not empty" check. Which, as this is a string constant, is always
true.

cmake/Version.cmake

@@ -126,7 +126,7 @@ else(FORCE_GIT_REVISION)
   find_program(GIT_EXECUTABLE git)
   message(STATUS "Found git: ${GIT_EXECUTABLE}")
 
-  if(NOT GIT_EXECUTABLE-NOTFOUND)
+  if(GIT_EXECUTABLE)
     execute_process(COMMAND ${GIT_EXECUTABLE} describe
         WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
         RESULT_VARIABLE GIT_INFO_RESULT