libdecnumber: Prepare libdecnumber for QEMU include structure

Consistent with other libraries in QEMU, the libdecnumber header files were
placed in include/libdecnumber, separate from the C code.  This is different
from the original libdecnumber source, where they were co-located.

Change the libdecnumber source code so that it reflects this split.  Specifically,
modify directives of the form:

    #include "xxx.h"

to look like:

    #include "libdecnumber/xxx.h"

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
diff --git a/libdecnumber/decNumber.c b/libdecnumber/decNumber.c
index f9a624a..1bfc081 100644
--- a/libdecnumber/decNumber.c
+++ b/libdecnumber/decNumber.c
@@ -170,9 +170,9 @@
 #include <stdio.h>		   /* for printf [if needed] */
 #include <string.h>		   /* for strcpy */
 #include <ctype.h>		   /* for lower */
-#include "dconfig.h"		   /* for GCC definitions */
-#include "decNumber.h"		   /* base number library */
-#include "decNumberLocal.h"	   /* decNumber local types, etc. */
+#include "libdecnumber/dconfig.h"
+#include "libdecnumber/decNumber.h"
+#include "libdecnumber/decNumberLocal.h"
 
 /* Constants */
 /* Public lookup table used by the D2U macro */