Fix install_subdir setup error

meson 0.60 gives the following error when attempting to setup:
meson.build:100:0: ERROR: install_subdir keyword argument "exclude_files" cannot be absolute

It's due to an internal change in string path construction.
See https://github.com/mesonbuild/meson/issues/9450 for a related bug
report.

Technically this behavior should go away with the next meson release,
but we can harden our own meson.build rather than rely on undocumented
behavior regarding whether or not the trailing slash will be removed.

Tested: `meson setup` is successful with meson 0.60

Change-Id: I34a919586975dffb4cfe71e77dcca31e5902af36
Signed-off-by: Jonathan Doman <jonathan.doman@intel.com>
diff --git a/meson.build b/meson.build
index 74c088a..7b2ad92 100644
--- a/meson.build
+++ b/meson.build
@@ -87,14 +87,14 @@
 generated_root = meson.current_build_dir() / 'gen/'
 exclude_cpp = []
 foreach f : generated_files_cpp
-    exclude_cpp += f.full_path().replace(generated_root, '')
+    exclude_cpp += f.full_path().replace(generated_root, '').strip('/')
 endforeach
 
 # Install the generated header files.
 exclude = exclude_cpp
 foreach o : generated_others
     foreach f : o.to_list()
-        exclude += f.full_path().replace(generated_root, '')
+        exclude += f.full_path().replace(generated_root, '').strip('/')
     endforeach
 endforeach
 install_subdir(
@@ -107,7 +107,7 @@
 # Install the generated markdown files.
 exclude = exclude_cpp
 foreach f : generated_files_headers
-    exclude += f.full_path().replace(generated_root, '')
+    exclude += f.full_path().replace(generated_root, '').strip('/')
 endforeach
 install_subdir(
     generated_root,