add -fPIC flag (position independent code) like in wlc

Linking fails otherwise:

Linking C executable ../bin/sway
/usr/bin/ld: CMakeFiles/sway.dir/commands.c.o: relocation R_X86_64_32 against `.rodata.str1.8' can not be used when making a shared object; recompile with -fPIC
CMakeFiles/sway.dir/commands.c.o: error adding symbols: Bad value
collect2: error: ld returned 1 exit status
sway/CMakeFiles/sway.dir/build.make:442: recipe for target 'bin/sway' failed

CMakeLists.txt

@@ -9,6 +9,7 @@ add_definitions(-DFALLBACK_CONFIG_DIR=\"${FALLBACK_CONFIG_DIR}\")
 set(CMAKE_C_FLAGS "-g")
 set(CMAKE_C_STANDARD 99)
 set(CMAKE_C_EXTENSIONS OFF)
+set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/bin)
 add_definitions(
     -D_GNU_SOURCE