os/binfmt/libelf: fix build warning
Add preprocessor condition to fix build warning as shown below
CC:  libelf/libelf_read.c
libelf/libelf_read.c: In function 'elf_read':
libelf/libelf_read.c:134:8: warning: unused variable 'rpos' [-Wunused-variable]
off_t rpos;     /* Position returned by lseek */
^~~~
@@ -131,7 +131,9 @@ static inline void elf_dumpreaddata(FAR char *buffer, int buflen)
int elf_read(FAR struct elf_loadinfo_s *loadinfo, FAR uint8_t *buffer, size_t readsize, off_t offset)
{
ssize_t nbytes; /* Number of bytes read */
+#if !defined(CONFIG_COMPRESSED_BINARY)
off_t rpos; /* Position returned by lseek */
+#endif
/* Advance offset by binary header size, loadinfo->offset will be 0 in normal exec call */
offset += loadinfo->offset;
