ust-compiler.h: Implement __LTTNG_COMPOUND_LITERAL

Implement __LTTNG_COMPOUND_LITERAL to allow defining a compound literal
with static storage.

Compound literals with static storage are needed by LTTng to implement
nested arrays and sequences in TRACEPOINT_EVENT.

Compound literals are part of the C99 and C11 standards, but not
part of the C++ standards. However, those are supported by both g++ and
clang as C99 extensions when compiling C++ code.

Strict C++11 compliance can be obtained by defining
LTTNG_ALLOCATE_COMPOUND_LITERAL_ON_HEAP before including this header
to allocate those on the heap in C++.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>