Add custom entyr to cuda buildtype dict. Closes #8336.

mesonbuild/compilers/compilers.py

diff --git a/mesonbuild/compilers/compilers.py b/mesonbuild/compilers/compilers.py
index 08db6d7..1872253 100644
--- a/mesonbuild/compilers/compilers.py
+++ b/mesonbuild/compilers/compilers.py
@@ -179,6 +179,7 @@
                        'debugoptimized': [],
                        'release': [],
                        'minsize': [],
+                       'custom': [],
                        }  # type: T.Dict[str, T.List[str]]
 java_buildtype_args = {'plain': [],
                        'debug': ['-g'],