Fix duplicate files being compiled

`config_file_userdata` and `string_list` were being compiled twice. Credit goes to @bparker06 for finding this.

Fixes #305

Makefile.common

@@ -42,11 +42,9 @@ SOURCES_C += $(wildcard \
 	$(CORE_DIR)/vendor/libretro-common/compat/fopen_utf8.c \
 	$(CORE_DIR)/vendor/libretro-common/compat/compat_posix_string.c \
 	$(CORE_DIR)/vendor/libretro-common/encodings/encoding_utf.c \
-	$(CORE_DIR)/vendor/libretro-common/file/config_file_userdata.c \
 	$(CORE_DIR)/vendor/libretro-common/file/config_file.c \
 	$(CORE_DIR)/vendor/libretro-common/lists/string_list.c \
 	$(CORE_DIR)/vendor/libretro-common/compat/compat_strcasestr.c \
-	$(CORE_DIR)/vendor/libretro-common/lists/string_list.c \
 )
 
 # stb_vorbis