fix: implicit-int error in EXPORT_SYMBOL_GPL

fix: implicit-int error in EXPORT_SYMBOL_GPL

Add module header to fix error:
error: type defaults to 'int' in declaration of 'EXPORT_SYMBOL_GPL'
[-Werror=implicit-int]

Log:
/workdir/build/workspace/sources/lttng-modules/src/wrapper/random.c:54:1:
warning: data definition has no type or storage class
   54 EXPORT_SYMBOL_GPL(wrapper_get_bootid);
      ^~~~~~~~~~~~~~~~~
/workdir/build/workspace/sources/lttng-modules/src/wrapper/random.c:54:1:
error: type defaults to 'int' in declaration of 'EXPORT_SYMBOL_GPL'
[-Werror=implicit-int]
/workdir/build/workspace/sources/lttng-modules/src/wrapper/random.c:54:1:
warning: parameter names (without types) in function declaration
cc1: some warnings being treated as errors
make[3]: ***
[/workdir/build/tmp/work-shared/qt5222/kernel-source/scripts/Makefile.build:271:
/workdir/build/workspace/sources/lttng-modules/src/wrapper/random.o]

Signed-off-by: Daniel Gomez <daniel@qtec.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: If1b96f6fcc19988b94dc09b12d071f2c61491b83