[LibOS/benchmark] Fix generation of SGX tokens

LibOS/shim/test/benchmark/Makefile

@@ -1,8 +1,8 @@
 c_executables = $(patsubst %.c,%,$(wildcard *.c))
 cxx_executables = $(patsubst %.cpp,%,$(wildcard *.cpp))
 
-target = $(c_executables) $(cxx_executables) \
-	  manifest
+exec_target = $(c_executables) $(cxx_executables)
+target = manifest
 
 include ../Makefile.Test