backtrace: Avoid potential spurious output.

clang 4.0 found this via -Wlogical-not-parentheses.

ASTERISK-27642

Change-Id: I9ec3e144d425a976c02811bd23cd0c533d2eca4e

main/backtrace.c

@@ -130,7 +130,7 @@ char **__ast_bt_get_symbols(void **addresses, size_t num_frames)
 			}
 
 			for (section = bfdobj->sections; section; section = section->next) {
-				if (!bfd_get_section_flags(bfdobj, section) & SEC_ALLOC ||
+				if (!(bfd_get_section_flags(bfdobj, section) & SEC_ALLOC) ||
 					section->vma > offset ||
 					section->size + section->vma < offset) {
 					continue;