[CMake] add extension to fix CMP0115

fix:

CMake Warning (dev) at unittest/CMakeLists.txt:10 (ADD_LIBRARY):
  Policy CMP0115 is not set: Source file extensions must be explicit.  Run
  "cmake --help-policy CMP0115" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  File:

    …/eigenpy/unittest/eigen_ref.cpp
Call Stack (most recent call first):
  unittest/CMakeLists.txt:33 (ADD_LIB_UNIT_TEST)
This warning is for project developers.  Use -Wno-dev to suppress it.

unittest/CMakeLists.txt

@@ -7,9 +7,9 @@ MACRO(ADD_LIB_UNIT_TEST test)
   CREATE_CTEST_BUILD_TESTS_TARGET()
 
   IF(BUILD_TESTING)
-    ADD_LIBRARY(${test} SHARED ${test})
+    ADD_LIBRARY(${test} SHARED "${test}.cpp")
   ELSE(BUILD_TESTING)
-    ADD_LIBRARY(${test} SHARED EXCLUDE_FROM_ALL ${test})
+    ADD_LIBRARY(${test} SHARED EXCLUDE_FROM_ALL "${test}.cpp")
   ENDIF(BUILD_TESTING)
 
   TARGET_LINK_LIBRARIES(${test} PUBLIC ${PROJECT_NAME})