perf build: Build error in libbpf missing initialization

In linux-next tree compiling the perf tool with additional make flags
EXTRA_CFLAGS="-Wp,-D_FORTIFY_SOURCE=2 -O2" causes a compiler error.
It is the warning 'variable may be used uninitialized' which is treated
as error: I compile it using a FEDORA 28 installation, my gcc compiler
version: gcc (GCC) 8.0.1 20180324 (Red Hat 8.0.1-0.20). The file that
causes the error is tools/lib/bpf/libbpf.c.

  [root@p23lp27] # make V=1 EXTRA_CFLAGS="-Wp,-D_FORTIFY_SOURCE=2 -O2"
  [...]
  Makefile.config:849: No openjdk development package found, please
     install JDK package, e.g. openjdk-8-jdk, java-1.8.0-openjdk-devel
  Warning: Kernel ABI header at 'tools/include/uapi/linux/if_link.h'
          differs from latest version at 'include/uapi/linux/if_link.h'
    CC       libbpf.o
  libbpf.c: In function ‘bpf_perf_event_read_simple’:
  libbpf.c:2342:6: error: ‘ret’ may be used uninitialized in this
  			function [-Werror=maybe-uninitialized]
    int ret;
        ^
  cc1: all warnings being treated as errors
  mv: cannot stat './.libbpf.o.tmp': No such file or directory
  /home6/tmricht/linux-next/tools/build/Makefile.build:96: recipe for target 'libbpf.o' failed

Suggested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -2273,8 +2273,8 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
 	volatile struct perf_event_mmap_page *header = mem;
 	__u64 data_tail = header->data_tail;
 	__u64 data_head = header->data_head;
+	int ret = LIBBPF_PERF_EVENT_ERROR;
 	void *base, *begin, *end;
-	int ret;
 
 	asm volatile("" ::: "memory"); /* in real code it should be smp_rmb() */
 	if (data_head == data_tail)