include: Declare ge25519_base as extern

Thanks to Tilo Eckart.

The global variable "ge25519_base" is referenced in the module
"src/external/ed25519.c" and initialized in "src/external/ge25519.c".
The lack of the extern keyword in the header results in different
instances being compiled into both translation units with some
compilers.

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/ge25519.h

@@ -28,7 +28,7 @@ typedef struct
   fe25519 t;
 } ge25519;
 
-const ge25519 ge25519_base;
+extern const ge25519 ge25519_base;
 
 int ge25519_unpackneg_vartime(ge25519 *r, const unsigned char p[32]);