Remaining fixes needed before running Tools/fix_headers.sh

<systemlib/err.h> --> "systemlib/err.h" that fix_headers.sh
would miss because it comes after code it doesn't understand.

Effectively remove the '__EXPORT extern perf_counter_t perf_alloc('
line, because currently perf_alloc is defined to be NULL, and
after running fix_headers.sh that happens *before* this header
is included (the order of headers will be changed).

Do not define NULL to be (void*)0: that only works for C.
In fact, the conversions needed for NULL are so full of exceptions
that standard C++ introduced a new *keyword*: nullptr.
That's what we really should be using for C++ code.
In this case I just include the correct header to define NULL
the correct way.

Not really related to the header line:
Removed an #include <time.h> because I noted that px4_time.h
was already included... and moved a #include <sys/time.h>
to the top of the file (not really a fan of including headers
in the middle unless absolutely necessary).
Removed a include of queue.h because I noted it wasn't used.

src/modules/systemlib/perf_counter.c

@@ -294,7 +294,7 @@ perf_end(perf_counter_t handle)
 	}
 }
 
-#include <systemlib/err.h>
+#include "systemlib/err.h"
 
 void
 perf_set_elapsed(perf_counter_t handle, int64_t elapsed)

src/modules/systemlib/perf_counter.h

@@ -64,7 +64,9 @@ __BEGIN_DECLS
  * @return			Handle for the new counter, or NULL if a counter
  *				could not be allocated.
  */
+#ifndef perf_alloc	// perf_alloc might be defined to be NULL in src/modules/px4iofirmware/px4io.h
 __EXPORT extern perf_counter_t	perf_alloc(enum perf_counter_type type, const char *name);
+#endif
 
 /**
  * Get the reference to an existing counter or create a new one if it does not exist.