CLEANUP: Add missing header to ssl_utils.c

Found with -Wmissing-prototypes:

    src/ssl_utils.c:22:5: fatal error: no previous prototype for function 'cert_get_pkey_algo' [-Wmissing-prototypes]
    int cert_get_pkey_algo(X509 *crt, struct buffer *out)
        ^
    src/ssl_utils.c:22:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
    int cert_get_pkey_algo(X509 *crt, struct buffer *out)
    ^
    static
    1 error generated.
diff --git a/src/ssl_utils.c b/src/ssl_utils.c
index 35c06f7..0f4a859 100644
--- a/src/ssl_utils.c
+++ b/src/ssl_utils.c
@@ -17,6 +17,7 @@
 #include <haproxy/chunk.h>
 #include <haproxy/openssl-compat.h>
 #include <haproxy/ssl_sock.h>
+#include <haproxy/ssl_utils.h>
 
 /* fill a buffer with the algorithm and size of a public key */
 int cert_get_pkey_algo(X509 *crt, struct buffer *out)