wasm_export.h: Use "default" visibility

Since the top-level CMakelists.txt is appending -fvisibility=hidden to
the compile options, no public symbols are exported by default. This
forbids users from linking against the shared library.

Using gcc/clang attributes [1], it is possible to override the
definition for WASM_RUNTIME_API_EXTERN so that only required symbols are
correctly exported.

[1]: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes

Author: midokura-xavi92

core/iwasm/include/wasm_export.h

@@ -23,6 +23,8 @@
 #else
 #define WASM_RUNTIME_API_EXTERN __declspec(dllimport)
 #endif
+#elif defined(__GNUC__) || defined(__clang__)
+#define WASM_RUNTIME_API_EXTERN __attribute__ ((visibility ("default")))
 #else
 #define WASM_RUNTIME_API_EXTERN
 #endif