Makefile.am: fixup ossfuzz LDADD

Automake cannot properly associate the libstandaloneengine.la target as
a dependency of the other ossfuzz targets, because it is specified with
the wrong path - leading to a make error when trying to build with
`--enable-ossfuzzers`.

Specify the correct path.

Signed-off-by: Alex Stewart <alex.stewart@ni.com>

Makefile.am

@@ -456,7 +456,7 @@ if USE_OSSFUZZ_STATIC
 FUZZ_LDADD = $(LIB_FUZZING_ENGINE)
 FUZZ_FLAG =
 else
-FUZZ_LDADD = libstandaloneengine.la
+FUZZ_LDADD = ossfuzz/libstandaloneengine.la
 FUZZ_FLAG =
 endif
 endif