elf.h: add DT_SYMTAB_SHNDX

it's a recent addition to elf gabi:
http://sco.com/developers/gabi/latest/revision.html
based on discussions at
https://sourceware.org/bugzilla/show_bug.cgi?id=15835

include/elf.h

@@ -742,7 +742,8 @@ typedef struct {
 #define DT_ENCODING	32
 #define DT_PREINIT_ARRAY 32
 #define DT_PREINIT_ARRAYSZ 33
-#define	DT_NUM		34
+#define DT_SYMTAB_SHNDX	34
+#define	DT_NUM		35
 #define DT_LOOS		0x6000000d
 #define DT_HIOS		0x6ffff000
 #define DT_LOPROC	0x70000000