Fix broken check for multi-process support

With the introduction of FreeRTOS support, the requirement that the
underlying platform supports multiple processes became untenable, and
therefore the code for dealing with multiple processes is now
automatically detected via some CMake code and then effectuated in the
sources via a macro (DDSRT_HAVE_MULTI_PROCESS).  Unfortunately, a typo
resulted in all platforms being treated as if without support for
multiple processes.  Fortunately, at this stage the only consequence was
the disabling of the first few multi-process tests.

Signed-off-by: Erik Boasson <eb@ilities.com>

src/ddsrt/cmake/process.c

@@ -11,7 +11,7 @@
  */
 #include "dds/ddsrt/process.h"
 
-#if DDSRT_HAVE_PROCESS
+#if DDSRT_HAVE_MULTI_PROCESS
 # error "cmake_HAVE_MULTI_PROCESS=true"
 #else
 # error "cmake_HAVE_MULTI_PROCESS=false"