Fix C compiler warning when using pre-C11 compiler (#57564)

e.g. when embedding Julia into another project:

warning: redefinition of typedef 'jl_genericmemory_t' is a C11 feature

Author: fingolfin

src/julia.h

@@ -66,7 +66,6 @@
 
 typedef struct _jl_taggedvalue_t jl_taggedvalue_t;
 typedef struct _jl_tls_states_t *jl_ptls_t;
-typedef struct _jl_genericmemory_t jl_genericmemory_t;
 
 #ifdef JL_LIBRARY_EXPORTS
 #include "uv.h"