efi_loader: correct includes in efi_variable.c

'make tests' on an 32bit ARM system leads to

In file included from ../lib/efi_loader/efi_variable.c:9:
../include/malloc.h:364:7: error: conflicting types for ‘memset’
 void* memset(void*, int, size_t);
       ^~~~~~
In file included from ../include/compiler.h:126,
                 from ../include/env.h:12,
                 from ../lib/efi_loader/efi_variable.c:8:
../include/linux/string.h:103:15:
note: previous declaration of ‘memset’ was here
 extern void * memset(void *,int,__kernel_size_t);
               ^~~~~~
In file included from ../lib/efi_loader/efi_variable.c:9:
../include/malloc.h:365:7: error: conflicting types for ‘memcpy’
 void* memcpy(void*, const void*, size_t);
       ^~~~~~
In file included from ../include/compiler.h:126,
                 from ../include/env.h:12,
                 from ../lib/efi_loader/efi_variable.c:8:
../include/linux/string.h:106:15:
note: previous declaration of ‘memcpy’ was here
 extern void * memcpy(void *,const void *,__kernel_size_t);
               ^~~~~~

Use common.h as first include as recommended by the U-Boot coding style
guide.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>