I know this thread is a bit stale, but I just had the same problem.
Basically you need to direct the compiler to the library linked file for emWin (since we don't have source code, only the linked libraries)
See the pdf component datasheet for the emWin Graphic Library -- on page 4
"Add the link library file. The emWin GUI can be used with the Keil_PK51, GNU CC, KEIL
MDK, or KEIL RVDS toolchain, so select the library according to the toolchain you choose.
From the menu Project > Build Settings > Linker > General > Additional Link Files,
select the library file.
Note The process of adding the library to the linker is different for the GCC toolchain. In this
case, the directory containing the library is added to the linker with the “Additional Library
Directories” setting and the library is specified with the “Additional Libraries” setting. The
library name used for this setting must have the “lib” prefix and “.a” suffix removed from the
name. In other words, give it the directory and in this case type in emWinnosnts under Additional Libraries