exec includedir should be pointing to general yocto includdir

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>

Index: gmp-6.1.2/Makefile.am
===================================================================
--- gmp-6.1.2.orig/Makefile.am
+++ gmp-6.1.2/Makefile.am
@@ -130,7 +130,7 @@ EXTRA_DIST += gmpxx.h
 # but anyone knowledgeable enough to be playing with exec_prefix will be able
 # to address that.
 #
-includeexecdir = $(exec_prefix)/include
+includeexecdir = $(includedir)
 include_HEADERS = $(GMPXX_HEADERS_OPTION)
 nodist_includeexec_HEADERS = gmp.h
 lib_LTLIBRARIES = libgmp.la $(GMPXX_LTLIBRARIES_OPTION)