I have tried to build the upf on the cascade lake server machine with Ubuntu 18.04.4 LTS and 5.0.0-23-generic kernel. I get the below error, looks in the build configuration there's some include missing to the kernel modules.

Making all in src
CC genl.lo
CC gtp5g-genl-pdr.lo
CC gtp5g-genl-far.lo
CC gtp5g-rtnl.lo
CC gtp5g.lo
gtp5g.c:71:14: warning: data definition has no type or storage class
EXPORT_SYMBOL(gtp5g_dev_alloc);
^
gtp5g.c:71:15: warning: type defaults to ‘int’ in declaration of ‘gtp5g_dev_alloc’ [-Wimplicit-int]
EXPORT_SYMBOL(gtp5g_dev_alloc);
^~~~~~~~~~~~~~~
gtp5g.c:71:15: error: ‘gtp5g_dev_alloc’ redeclared as different kind of symbol
gtp5g.c:60:19: note: previous definition of ‘gtp5g_dev_alloc’ was here
struct gtp5g_dev *gtp5g_dev_alloc(void)
^~~~~~~~~~~~~~~
gtp5g.c:48:14: warning: data definition has no type or storage class
EXPORT_SYMBOL(GTP5G_STRUCT_FUNC_NAME(_func_name))
^
gtp5g.c:73:1: note: in expansion of macro ‘gtp5g_struct_alloc_exp’
gtp5g_struct_alloc_exp(pdr, struct gtp5g_pdr);
^~~~~~~~~~~~~~~~~~~~~~
gtp5g.c:38:35: warning: type defaults to ‘int’ in declaration of ‘gtp5g_pdr_alloc’ [-Wimplicit-int]
#define GTP5G_STRUCT_FUNC_NAME(x) gtp5g ##x## _alloc

Attaching the generated build folder content

build.zip