Avoid an implicit int, to prevent build failures with future compilers.

Submitted upstream: <https://bugs.passt.top/show_bug.cgi?id=42>


diff --git a/Makefile b/Makefile
index 1dc2df5884d3b597..cd62900e7406e155 100644
--- a/Makefile
+++ b/Makefile
@@ -95,7 +95,7 @@ ifeq ($(shell :|$(CC) -fstack-protector-strong -S -xc - -o - >/dev/null 2>&1; ec
 	FLAGS += -fstack-protector-strong
 endif
 
-C := \#define _GNU_SOURCE\n\#include <fcntl.h>\nx = FALLOC_FL_COLLAPSE_RANGE;
+C := \#define _GNU_SOURCE\n\#include <fcntl.h>\nint x = FALLOC_FL_COLLAPSE_RANGE;
 ifeq ($(shell printf "$(C)" | $(CC) -S -xc - -o - >/dev/null 2>&1; echo $$?),0)
 	EXTRA_SYSCALLS += fallocate
 endif