hash: fix build failure against gcc-10

On gcc-10 (and gcc-9 -fno-common) build fails as:

```
gcc  -g -O2 -pthread -pthread -Wall -Werror -pedantic -Wmissing-prototypes \
  -Wmissing-declarations -Wredundant-decls   -o memcached ... -levent
ld: memcached-hash.o:memcached/hash.h:5:
  multiple definition of `hash'; memcached-memcached.o:memcached/hash.h:5: first defined here
```

gcc-10 will change the default from -fcommon to fno-common:
https://gcc.gnu.org/PR85678.

The error also happens if CFLAGS=-fno-common passed explicitly.

Reported-by: Brian Evans
Bug: https://bugs.gentoo.org/706196
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/hash.h b/hash.h
index 059d1e2..3b2a984 100644
--- a/hash.h
+++ b/hash.h
@@ -2,7 +2,7 @@
 #define    HASH_H
 
 typedef uint32_t (*hash_func)(const void *key, size_t length);
-hash_func hash;
+extern hash_func hash;
 
 enum hashfunc_type {
     JENKINS_HASH=0, MURMUR3_HASH