Introduce an `include` directory for header files

Move all header files to the new `include` directory.
Adapt include directives and build systems to the new directory.

Change-Id: Iaddd6bbfa0624b166d422f665877f096983bf4cf
Signed-off-by: Felix Singer <felix.singer@secunet.com>
Signed-off-by: Thomas Heijligen <thomas.heijligen@secunet.com>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/58622
Original-Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/72322
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Felix Singer <felixsinger@posteo.net>
diff --git a/meson.build b/meson.build
index be61892..1e571f5 100644
--- a/meson.build
+++ b/meson.build
@@ -358,10 +358,12 @@
 mandir = join_paths(prefix, get_option('mandir'))
 
 install_headers([
-    'libflashrom.h',
+    'include/libflashrom.h',
   ],
 )
 
+include_dir = include_directories('include')
+
 # core modules needed by both the library and the CLI
 srcs += '82802ab.c'
 srcs += 'at45db.c'
@@ -401,6 +403,7 @@
   sources : [
     srcs,
   ],
+  include_directories : include_dir,
   soversion : lt_current,
   version : lt_version,
   dependencies : [
@@ -444,7 +447,7 @@
 
 flashrom_dep = declare_dependency(
   link_with : flashrom,
-  include_directories : include_directories('.'),
+  include_directories : include_dir,
   dependencies : deps
 )
 
@@ -471,6 +474,7 @@
     'cli_output.c',
     'flashrom.c',
   ],
+  include_directories : include_dir,
   dependencies : [
     deps,
   ],