Wrap bootcamp DEBUG statement with curly braces

This fixes a -Wempty-body warning with gcc 6.3.0 when PART_DEBUG is undefined.

MFC after:	3 days
Reported by:	Jenkins (FreeBSD-head-amd64-gcc job)
Tested with:	amd64-gcc-6.3.0 (devel/amd64-xtoolchain-gcc)
Sponsored by:	Dell EMC Isilon

sys/boot/common/part.c

@@ -684,8 +684,9 @@ ptable_open(void *dev, uint64_t sectors, uint16_t sectorsize,
 		if (dp[1].dp_typ != DOSPTYP_HFS) {
 			table->type = PTABLE_NONE;
 			DEBUG("Incorrect PMBR, ignore it");
-		} else
+		} else {
 			DEBUG("Bootcamp detected");
+		}
 	}
 #ifdef LOADER_GPT_SUPPORT
 	if (table->type == PTABLE_GPT) {