apex: only mount required apex files

Since recent kernels seem to limit the number of loopback
devices to 7, we now just mount the required apex files in TWRP.

To mount additional apex files specify TW_ADDITIONAL_APEX_FILES
in your BoardConfig, for example:
TW_ADDITIONAL_APEX_FILES := "apex1 apex2"

To disable Apex in your builds use:
TW_EXLUCDE_APEX := true
Change-Id: Ib55529a4dc17ce2b737b01b86100dca3dc75e6c9

Change-Id: I3b4dfbb164838ffb126016b0d862f67d3f170bf3
diff --git a/twrpApex.hpp b/twrpApex.hpp
index f42ed8a..edd1d5f 100755
--- a/twrpApex.hpp
+++ b/twrpApex.hpp
@@ -5,6 +5,7 @@
 #include <string>
 #include <vector>
 #include <filesystem>
+#include <sstream>
 
 #include <sys/types.h>
 #include <sys/stat.h>