build: fix build with CMake 2

Found that some package builds failed on the mistake in CMakeLists.txt
file, the failed packages and test builds were:
- CentOS 6
- CentOS 7
- Ubuntu 14.04
and static build based on Dockerfile.

The core of the issue is that CMake 2 does not support line continuation
with backslash.

The commit fixes the regression from
7b443650 ('feedback: add cmake option to
disable the daemon').

Follow up #3308

src/box/CMakeLists.txt

@@ -3,8 +3,7 @@ file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/src/box/lua)
 # Sometimes 'spying' code is not acceptable even if it would be
 # disabled by default. That option allows to remove the feedback
 # daemon from the build completely.
-option(ENABLE_FEEDBACK_DAEMON "Feedback daemon which reports debug data to \
-the Tarantool team" ON)
+option(ENABLE_FEEDBACK_DAEMON "Feedback daemon which reports debug data to the Tarantool team" ON)
 
 add_subdirectory(sql)